◉[Capitulum 1]

◉[Chapter 1]⁑

4.1 ◉ Invenimus visum quando inspexerit luces valde fortes fortiter dolebit ex eis et habebit nocumentum, aspiciens enim quando aspexerit corpus solis non poterit bene aspicere ipsum, quoniam visus eius dolebit propter eius lucem. Et similiter quando inspexerit speculum tersum super quod ascendebat splendor solis et fuerit visus eius in loco ad quem reflectitur lux ab illo speculo, dolebit etiam propter lumen reflexum perveniens ad suum visum a speculo, et non poterit aperire oculum ad inspiciendum illud lumen.

4.1 ◉ We find that when our sight fixes upon very strong light-sources it will suffer intense pain and impairment from them, for when an observer looks at the body of the sun, he cannot do so properly because his vision will suffer from its light. By the same token, when he looks at a polished mirror flooded with sunlight, and his eye is placed at the spot to which the light from that mirror is reflected,⁑ his vision will also suffer from the reflected light reaching his eye from the mirror, and he cannot open his eye to look at that light.⁑

4.2 ◉ Et invenimus etiam quando aspiciens intuetur corpus mundum album super quod ascendebat lux solis et moretur in aspectu ipsius, deinde auferat visum suum ab eo ad locum obscurum debilis lucis, quod fere non poterit comprehendere res visibiles illius loci comprehensione vera, et inveniet coopertorium quasi inter ipsum et ipsas. Deinde paulatim discooperietur, et revertetur visus in suam dispositionem. Et iterum quando inspiciens inspexerit fortem ignem et fuerit intuens ipsum et moretur in aspiciendo longo tempore, deinde declinet visum suum ad locum obscurum debilis lucis, et inveniet etiam idem in visu suo.

4.2 ◉ Furthermore, we find that when an observer stares at a pure white body illuminated by sunlight, and keeps staring for awhile, then shifts his focus from it to a dark, dimly lit location, he can scarcely make out the visible objects at that location. Instead, it will seem to him as if there were a screen between himself and them. Then, after awhile his vision will clear up and return to its normal state. So too, when an observer looks at a strong fire and continues to stare at it for a long time, if he then shifts his focus to a dark, dimly lit location, he will experience the same visual effect.

4.3 ◉ Et iterum invenimus quando inspiciens inspexerit corpus mundum album super quod oriebatur lux diei et fuerit illa lux fortis, quamvis non sit lux solis, et moretur in aspectu diu, deinde auferat visum suum ad locum obscurum, inveniet formam illius lucis in loco obscuro illo, et inveniet cum hoc figuram eius. Deinde si clauserit visum et fuerit intuens secundum horam, inveniet in oculo suo formam illius lucis. Deinde auferetur hoc, et revertetur visus in suam dispositionem. Et simili ter erit dispositio visus quando inspexerit corpus super quod oriebatur lux solis.

4.3 ◉ We also find that, when an observer looks at a pure white body illuminated by intense daylight, even though there may be no [direct] sunlight, if he continues to look at that body for awhile and then shifts his focus to a dark location, he will see the form of its light, along with its shape, in that dark location. If he then closes his eyes and stares for a time, he will see the form of that light in his eye. In time this effect will wear off, and his vision will return to its normal state. The same thing will happen to his vision when he stares at an object illuminated by sunlight.⁑

4.4 ◉ Et similiter si inspexerit corpus clare album super quod oriebatur lux ignis, quando lux ignis fuerit fortis, et moretur in aspiciendo ipsum, deinde recesserit ad locum obscurum, inveniet in eo etiam idem hoc in suo visu. Et similiter quando aspiciens fuerit in domo in qua fuerit foramen amplum discoopertum ad celum et aspexerit ex illo loco celum in luce diei et moretur in aspiciendo ipsum, deinde revertatur visus eius ad locum obscurum in domo, inveniet formam lucis quam comprehendebat ex foramine cum figura foraminis in loco obscuro. Et si clauserit oculum suum, inveniet etiam in eo formam illam.

4.4 ◉ Likewise, if he looks at a bright white body illuminated by strong firelight and continues to stare at it, then refocuses on a dark location, he will experience the same visual effect. So too, when an observer is in a room with a large window open to the sky and continues to stare out at the sky during daylight, then shifts his focus to a dark spot in the room, he will see the form of the light that he perceived through the window along with the shape of the window in that dark spot. And if he closes his eye, he will also see that form in it.

4.5 ◉ Omnia ergo ista significant quod lux operetur in visum aliquam operationem.

4.5 ◉ All of these occurrences therefore indicate that light may affect vision in some way.

4.6 ◉ Et invenimus etiam inspicientem quando inspexerit viridarium multe spissitudinis herbarum super quod oriebatur lux solis et moretur in aspiciendo ipsum, deinde auferat visum suum ad locum obscurum, inveniet in illo loco obscuro formam illius lucis coloratam a virore illarum herbarum. Deinde si aspexerit in ista dispositione visibilia alba et fuerint illa visibilia in umbra et loco debilis lucis, inveniet colores illos admixtos cum virore. Et si clauserit oculum suum etiam et fuerit intuens in eo, inveniet in suo oculo formam lucis et formam viroris. Deinde discooperietur illud et auferetur. Et similiter si aspexerit corpus coloratum colore lazuleo vel rubeo vel alio colore forti scintillante super quod oriebatur lux solis et moretur in aspiciendo ipsum, deinde auferat visum suum ad visibilia alba in loco debilis lucis, inveniet colores eorum admixtos cum illo colore.

4.6 ◉ And we also find that when an observer looks at a thickly planted garden illuminated by sunlight and continues to stare at it, then shifts his focus to a dark location, he will see the form of that light tinged by the green of those plants in that dark location. Afterward, under the same circumstances, if he stares at white objects lying in shadow or in a weakly illuminated location, he will see those colors mixed with green. And if he closes his eye and stares, he will see the form of the light as well as the form of green in his eye.⁑ In time this effect will clear up and disappear. Likewise, if he looks at an object that is colored azure or red or any other bright hue illuminated by sunlight and continues to stare at it, then shifts his focus to white objects lying in a dimly lit location, he will find their colors mixed with the original hue.

4.7 ◉ Ista ergo significant quod colores illuminati operentur in visum.

4.7 ◉ These instances therefore indicate that illuminated colors may affect vision.

◉[Capitulum 2]

◉[Chapter 2]⁑

4.8 ◉ Et etiam videmus stellas in nocte, et non videmus ipsas in luce diei; et nulla differentia est inter duo tempora nisi quod aer medians inter visum nostrum et celum est in die illuminatus et in nocte obscurus. Dum ergo aer fuerit obscurus, nos videmus stellas; cum autem illuminatus fuerit aer medius inter visum nostrum et stellas, latebunt nos stelle.

4.8 ◉ In addition, we see the stars at night but do not see them in daylight; and the only difference between the two times is that the intervening air between our eyes and the sky is illuminated during the day and dark at night. Hence, while the air is dark, we see the stars; but when the intervening air between our eyes and the stars is illuminated, the stars will be invisible to us.⁑

4.9 ◉ Et similiter si aspiciens fuerit de nocte aspiciens in loco luminoso lumine ignis, et fuerit lumen ignis extensum super terram, et fuerint in illo loco visibilia subtilia aut visibilia in quibus sunt res subtiles, et fuerint in aliqua umbra sed non forti, et non fuerit ignis medius inter visum et illa visibilia, et fuerit tunc inspiciens comprehendens illa visibilia et res subtiles que sunt in eis, deinde moveatur a suo loco donec sit ignis medius inter visum suum et illa visibilia. Tunc illa visibilia latebunt ipsum, si fuerint subtilia, vel subtilia que in eis sunt, et fere non comprehendet ipsa cum ignis fuerit medius inter visum suum et ipsa visibilia. Et si cooperiatur ignis a visu suo, comprehendet statim illa visibilia que latebant ipsum; et si auferatur coopertorium inter visum suum et ignem, latebunt ipsum iterum illa visibilia.

4.9 ◉ Likewise, suppose that an observer looks during the night toward a location illuminated by the light of a fire and that the firelight shines upon the ground; suppose also that there are tiny objects or objects with subtle⁑ features in that location and that they lie in shadow that is not too intense; and suppose that the fire is not interposed between the observer’s eyes and those objects and, accordingly, that the observer makes those objects out as well as the subtle features possessed by them. Let him then move from his [original] position until the fire is situated between his eyes and those objects. In that case, neither the objects [themselves], if they are tiny, nor the subtle features possessed by them will be visible to him, and he will scarcely make them out when the fire lies between his eyes and those objects. If, however, the fire is screened from his line-of-sight, he will immediately make out those objects that had been invisible to him; but if the screen between his eyes and the fire is lifted, those objects will again be invisible to him.

4.10 ◉ Iste ergo dispositiones significant quod luces fortes orientes super visum et super aerem inter oculum et rem visam prohibent visum a comprehensione quorumdam visibilium quorum luces sunt debiles.

4.10 ◉ These situations therefore indicate that intense lights that shine upon the eyes and upon the air lying between the eyes and the visible object prevent the sight from making out certain objects that are dimly illuminated.

4.11 ◉ Et iterum quando aspiciens aspexerit corpus tersum et fuerint in illo corpore sculpture subtiles et non fuerint ille sculpture diversorum colorum a colore corporis sed fuerint ex colore illius corporis, et fuerit aspiciens in loco temperate lucis et fuerit ille locus oppositus soli vel quibusdam parietibus illuminatis lumine forti, deinde oppositum fuerit celo aut parieti illuminato, reflectetur ab eo aliqua lux ad visum, et inveniet aspiciens lucem apparentem in superficie corporis et in loco a quo reflectitur lux fortiorem et magis scintillantem. Et in ista dispositione, si inspiciens fuerit intuens illud corpus tersum, non videbit in eo aliquam sculpturam ex sculpturis que sunt in loco lucis fortis scintillantis illius corporis. Deinde si inspiciens inclinaverit illud corpus ab illo loco ita quod reflexio fiat ad alium locum extra locum visus sui, et fuerit cum hoc super corpus illud lux temperata, tunc inspiciens comprehendet sculpturas que sunt in eo quas non comprehendebat in reflexione lucis a corpore ad suum visum.

4.11 ◉ Furthermore, when an observer looks at a polished body on which there are subtle engravings that are not of a different color but rather of the same color as the body, and when the observer is in a moderately lit place, and this place faces the sun or some walls that are illuminated by intense light, then, when that object faces the sky or the illuminated wall, some light will be reflected from it to the eye, and the observer will find the light that appears on the body’s surface, as well as at the spot where the light reflects, to be quite intense and brilliant. Moreover, if the observer looks at that polished body under these circumstances, he will see none of the engravings in it where the intense, brilliant light is. Afterward, if the observer inclines that body away from the [original] location so that the reflection takes place to another spot outside the location of his eyes, and if in this case a moderate light shines upon that body, then the observer will make out the engravings in it that he had not made out when the light was reflected from the body to his eyes.

4.12 ◉ Et similiter quando lux reflectetur a pagina tersa in qua sunt sculpture subtiles ad visum, non distinguet visus illas sculpturas nec verificabit donec sit lux reflexa ad visum ab illa pagina. Et si declinetur superficies pagine ita quod situs eius mutetur et non reflectatur lux ab ea ad visum, comprehendet tunc visus illas sculpturas et distinguet.

4.12 ◉ By the same token, when light reflects to the eyes from a smooth page with subtle tracings on it, sight will not discern those tracings, nor will it perceive them distinctly as long as the light is reflected from that page to the eyes. But if the surface of the page is slanted so that its position is changed and the light no longer reflects from it to the eyes, then the visual faculty will make out those tracings and will perceive them distinctly.⁑

4.13 ◉ Et iterum quando ignis debilis fuerit in lumine debili, apparebit et comprehendetur a visu, et cum fuerit in lumine solis, apparebit corpus in quo est densum coloratum colore scintillante forti.

4.13 ◉ Likewise, when there is a low fire in a dimly lit place, it will be visible and will be made out by sight, but when it lies in sunlight, the object that is on fire will appear as a solid body that is colored with a very bright hue.

4.14 ◉ Et si positum fuerit prope illud corpus corpus album clare albedinis et fuerit illud corpus in umbra et in luce debili, apparebit super ipsum color corporis, sicut narravimus superius. Deinde si appropinquet illud corpus album donec sit in lumine solis, latebit ille color qui est super eum, et si revertatur ad umbram, apparebit ille color fulgens super ipsum. Et apud suum esse in luce forti et apud latitationem corporis quod est super ipsum, si obumbretur corpus corpore denso et sit in suo loco donec debilitetur lux que est super ipsum, apparebit color qui est super ipsum. Et si auferatur corpus obumbrans donec vigorescat lux super corpus album, latebit color qui est super ipsum.

4.14 ◉ And if a bright white body is placed next to that [burning] body, and if that [white] body lies in shadow or is dimly illuminated, the color of the [burning] body will appear on it, as we discussed earlier.⁑ Then, if that white body is brought out into sunlight, the color that shines upon it will disappear, but if it is brought back into the shadow, that color will appear shining upon it. And if the white body lies in strong light so that the [color of the other] body no longer appears upon it, but if that [white] body is shaded by a solid body and remains in place while the light that shines upon it is attenuated, the color that shines upon it will [re]appear. And if the shading body is removed so that the light shining upon the white body intensifies, the color shining on it will disappear.

4.15 ◉ Et similiter quando appropinquaverimus corpus diafonum coloratum colore scintillante igni vehementer forti et appropinquaverimus umbre illius corporis pannum album, apparebit color illius corporis diafoni super illum pannum, sicut narravimus prius. Deinde si appropinquaverimus illi panno alium ignem ita quod lux eius oriatur super illum pannum, latebit ille color qui apparebat super pannum, et non apparebit nisi albedo panni tantum. Et si auferamus illum ignem secundus, apparebit color super pannum.

4.15 ◉ Likewise, when we bring a brightly colored, transparent body next to a roaring fire and place a white cloth in the shadow of that body, the color of that transparent body will shine upon that cloth, as we pointed out earlier.⁑ Then, if we bring another fire next to that cloth so that its light shines upon that cloth, the color that appeared upon the cloth will disappear, and only the white of the cloth will be seen. But if we remove that second fire, the color will [re]appear upon the cloth.

4.16 ◉ Et etiam quedam animalia marina habent conchas et telas, et cum fuerint in loco obscuro in quo non est lux, apparebunt ille conche quasi ignis; et si inspiciens inspexerit eas in luce diei vel in luce ignis, comprehendet eas et non videbit in eis lumen vel aliquem ignem. Et similiter quando animal quod dicitur noctiluca volat de nocte, apparet quasi lampas, et cum aspiciens inspexerit eum in luce diei vel in luce ignis, apparebit animal sine igne.

4.16 ◉ Also, certain marine animals have shells or membranes that will appear incandescent when they are in a dark location without light; but if an observer looks at them in daylight or in firelight, he will perceive them but will see no light or fire in them. By the same token, when the animal that is called a »firefly« flits about at night, it looks like a lamp, but when an observer examines it in daylight or in firelight, the animal will appear without fire.⁑

4.17 ◉ Significant ergo omnes iste dispositiones quas declaravimus quod luces fortes visibilium aliquando occultant res que sunt in quibusdam visibilibus et quod luces debiles ali quando manifestant quasam res que sunt in quibusdam visibilibus.

4.17 ◉ Accordingly, all of the situations that we have detailed indicate that intensely luminous objects sometimes occlude features possessed by various visible objects, whereas feeble illumination sometimes reveals certain features possessed by various visible objects.

◉[Capitulum 3]

◉[Chapter 3]

4.18 ◉ Et iterum visui multotiens latent multe res que sunt invisibiles ex sculpturis subtilibus et scripturis subtilibus quando fuerint in lucibus debilibus vel in locis obscuris, et cum extrahuntur ad loca luminosa fortis luminis vel ponuntur in luce solis, apparebunt res que sunt in eis que latebant in locis obscuris et in lucibus debilibus. Et similiter sculpture subtiles, nequit visus comprehendere earum comprehensiones in locis obscuris et in lucibus debilibus; et cum extrahuntur ad luces fortes, comprehenduntur a visu.

4.18 ◉ Oftentimes several characteristics of subtle tracings or tiny writing are invisible to sight when they are in dimly lit or dark locations, whereas, when they are brought out into intensely illuminated locations or are placed in sunlight, those features of theirs that were invisible in the dark or in feeble light will appear. Likewise, sight is incapable of making out subtle tracings in dark places or in feeble light; but when they are brought out into strong light, they are made out by sight.

4.19 ◉ Significatur ergo per hanc dispositionem quod luces fortes manifestant multas res visibilium et quod luces debiles occultant multas res visibiles.

4.19 ◉ It is therefore shown by this example that strong light reveals many features of visible objects and that feeble light occludes many visible features.⁑

◉[Capitulum 4]

◉[Chapter 4]

4.20 ◉ Et iterum invenimus multa corpora densa colorata coloribus scintillantibus, sicut lazuleis, et vinosis, et celestibus, quando fuerint in locis obscuris et lucibus debilibus, apparebunt colores eorum turbidi. Et cum fuerint in luce forti apparebunt colores eorum scintillantes clari, et quanto augmentabitur lux super ipsum tanto augmentabitur super ipsum scintillatio coloris et claritas. Et cum fuerit aliquod istorum corporum in loco obscuro et non fuerit in eis nisi lux parva valde, illud corpus apparebit obscurum, et non distinguet visus colorem eius, et videbitur quasi niger. Et cum extrahitur ad loca luminosa lumine forti, apparebit color eius et distinguetur a visu.

4.20 ◉ Furthermore, we find that many solid bodies that are tinged with such bright colors as azure, wine-red, or sky-blue appear of a dull color when they are in dark or dimly lit locations.⁑ But when they lie in strong light, their colors will appear bright and clear, and the more intense the light shining upon them, the brighter and clearer their color will be. And when any of these bodies is placed in a dark location with very little light, that body will appear dark, sight will not discern its color, and it will appear black. But when it is brought out into intensely illuminated locations, its color will appear and will be discerned by sight.

4.21 ◉ Et invenimus etiam corpora turbidi coloris quod, quando lux oritur super ipsa fortis, quod colores eorum clarescunt; et invenimus etiam quod, quando lux fortis oritur super corpora densa alba, augmentabuntur in albedine et scintillatione apud sensum.

4.21 ◉ We also find that, when strong light shines upon bodies whose colors are dull, their colors brighten; and we also find that when strong light shines upon solid white bodies, their whiteness and brightness will be sensibly increased.

4.22 ◉ Et etiam invenimus corpora diafona colorata coloribus fortibus, sicut vina fortia fortis ruboris que sunt in vasis diafonis, quando fuerint in locis obscuris et lucibus debilibus, apparebunt nigra et obscura et quasi non diafona. Et cum fuerint in lucibus fortibus et oriatur super ipsa lux solis, clarescent colores eorum, et apparebit in eis diafonitas.

4.22 ◉ So too we find that, when intensely colored transparent objects, such as robust wines of deep redness that are in transparent vessels, are in dark or dimly lit locations, they will appear black and dark, as if they were not transparent. But when they are in strong light or flooded by sunlight, their colors will brighten, and their transparency will become apparent.

4.23 ◉ Et similiter lapides diafoni colorati, quando fuerint in locis obscuris, apparebunt colores eorum turbidi et obscuri; et cum super ipsos oritur lux fortis vel ponuntur in oppositione lucis ita quod lux pertranseat ipsos, apparebunt colores eorum clari, et apparebit in eis diafonitas propter penetrationem lucis.

4.23 ◉ Likewise, when transparent colored stones are in dark locations, their colors will appear dull and dark; but when intense light shines upon them, or when they are placed against a light-source so that its light shines through them, their colors will appear bright, and their transparency will be revealed by the passage of light [through them].

4.24 ◉ Et etiam quando corpora diafona colorata ponuntur in oppositione lucis et fuerit positum ex parte contraria parti lucis corpus album, sicut diximus superius, si lux fuerit fortis, apparebit forma illius coloris in umbra eius super corpus album oppositum ei. Et si lux oriens super ipsum fuerit debilis, apparebit super corpus album oppositum ei umbra tantum, et non apparebit color.

4.24 ◉ Furthermore, when colored transparent objects are put against the light and a white object is placed [facing them] on the side opposite the light, then, as we described it above,⁑ if the light is intense, the form of that color will appear in the shadow cast upon the facing white object. But if the light shining on the transparent object is feeble, only its shadow, not its color, will appear on the facing white object.

4.25 ◉ Et iterum invenimus pennas pavonis et pannum qui dicitur amilialmon quod diversatur in colore apud visum in diversis temporibus diei secundum diversitatem lucis orientis super ipsa.

4.25 ◉ In addition, we find that peacock feathers and the cloth called »amilialmon«⁑ vary in color according to sight at different times of the day, depending on how the light shines upon them.

4.26 ◉ Significant ergo iste dispositiones apparentes in coloribus quod colores corporum coloratorum non comprehenduntur a visu nisi secundum luces orientes super ipsa.

4.26 ◉ These phenomena involving color therefore indicate that the way the colors of tinted bodies are perceived by sight depends entirely upon the light that shines upon them.⁑

◉[Capitulum 5]

◉[Chapter 5]

4.27 ◉ Et cum luces fortes visibilium occultent quasdam res que sunt in quibusdam visibilibus aliquando et aliquando manifestent res quasdam que sunt in quibusdam visibilibus, et luces debiles visibilium aliquando manifestant quasdam res que sunt in quibusdam visibilibus et aliquando occultant quasdam res que sunt in quibusdam visibilibus, et corporum coloratorum colores aliquando alterantur secundum diversitatem lucis que oritur super ipsa, et luces fortes orientes super visum aliquando prohibent visum a comprehensione quorumdam visibilium, et visus tamen in omnibus istis nichil comprehendit ex visibilibus nisi sit illuminata, forma ergo quod comprehendit visus ex re visa non est nisi secundum lucem que est in illa re visa, et secundum luces que oriuntur super visum a comprehensione illius rei visibilis, et super aerem medium inter visum et rem visam.

4.27 ◉ And since strong light [shining] from visible objects at times occludes certain features possessed by some visible entities and at times reveals certain features possessed by some visible entities, and since feeble light [shining] from visible objects at times reveals certain features possessed by some visible entities and at times occludes certain features possessed by some visible entities, and since the colors of tinted objects are sometimes altered by variation in the light that shines upon them, and since strong light shining upon the eye sometimes prevents sight from making out certain visible objects, and since in all these instances sight nonetheless perceives nothing about visible objects unless they are illuminated, the form of the visible object that sight perceives depends entirely upon the light possessed by that visible object, as well as upon the light that shines upon the eyes when that visible object is perceived, and upon [the light that illuminates] the aerial medium between the eyes and the visible object.

4.28 ◉ Quare vero luces fortes prohibent visum a comprehensione quorumdam visibilium erit declaratum a nobis apud sermonem nostrum in qualitate visionis.

4.28 ◉ Why, however, strong light prevents sight from perceiving certain visible objects will be shown by us when we discuss the way in which vision is carried out.⁑

◉[Capitulum 6]

◉[Chapter 6]⁑

5.1 ◉ Oculus autem est compositus ex telis et corporibus diversis, et principium et incrementum eius est ex anteriori cerebri.

5.1 ◉ The eye is in fact composed of various membranes and bodies, and its origin and wellspring lie at the front of the brain.

5.2 ◉ Quoniam ex anteriori crescunt duo nervi obtici consimiles, et incipiunt oriri ex duobus locis a duabus partibus anterioris cerebri. Et dicitur quod uterque illorum habet tunicas et quod illi crescunt a duabus telis cerebri et perveniunt ad medium exterioris partis cerebri et anterioris. Deinde concurrunt et efficiunt unum nervum obticum; deinde iste nervus dividitur et efficiuntur iterum duo nervi obtici equales consimiles. Deinde extenduntur isti duo nervi donec perveniant ad duo convexa duorum ossium concavorum continentium oculos.

5.2 ◉ For two matching hollow⁑ nerves emerge from the front [of the brain], each arising from a spot on one of the two sides of the anterior part of the brain. And it is said that each of them has tunics and that they both arise from the two membranes of the brain and reach the middle of the outer surface of the front of the brain. They then intersect and form a single hollow nerve, after which this nerve splits, and they again form two matching and equal hollow nerves. Finally, these two nerves continue until they reach both cavities of the two eyesockets that contain the eyeballs.⁑

5.3 ◉ Et in duobus mediis istorum duorum concavorum ossium sunt duo foramina equaliter perforata, et situs eorum ex nervo communi est situs consimilis. Illi ergo nervi intrant ista foramina duo et exeunt ad concava duorum ossium, et illic dilatantur et ampliantur, et efficitur extremitas utriusque eorum quasi instrumentum ponendi vinum in doleis. Et uterque oculorum est compositus super istam extremitatem nervi que est sicut rameh, scilicet predictum instrumentum, et consolidatur cum ipso; et situs utriusque oculorum ex nervo communi est situs consimilis.

5.3 ◉ In the center of both of these eyesockets lie two openings of equal size, each one similarly disposed in relation to the common nerve. The [two] nerves therefore pass through these two openings and come out into the cavity of the two eye sockets where they expand and enlarge, and the endpoint of each of them forms something like the utensil used for pouring wine into jars. And each eye is attached to this endpoint on the nerve, which is like a funnel—i.e., the aforementioned utensil—and it forms a whole with it; and the location of each eye is the same in relation to the common nerve.

5.4 ◉ Et totus uterque oculus est compositus ex tunicis multis.

5.4 ◉ And each eye as a whole is composed of several tunics.

5.5 ◉ Prima ergo illarum est pinguedo alba que implet concavum ossis, et est maximum oculi, et dicitur consolidativa.

5.5 ◉ Accordingly, the first of these tunics is a white fat that fills the cavity of the bone, and it forms the majority of the eye and is called the sclera.⁑

5.6 ◉ Et intra istam pinguedinem est spera rotunda concava nigra pluries et viridis et glauca in quibusdam oculis, et corpus istius spere est tenue et cum hoc densum et non rarum. Et manifestum eius est applicatum cum consolidativa, et interius eius est concavum; et in parte concavitatis est quasi quedam attritio. Et quasi consolidativa continet istam speram preter quam suum anterius quoniam consolidativa non cooperit anterius istius spere sed circulatur super anterius eius. Et ista tela dicitur uvea quia assimulatur uve.

5.6 ◉ And inside this [outer tunic of] fat is a round, concave sphere that is generally black, but green⁑ or grey in some eyes, and the body of this sphere is thin yet nonetheless solid rather than loosely textured. And its outer surface is attached to the sclera, while its inner surface is concave; and on its concave side there is a sort of roughness. The sclera surrounds all but the anterior part of this sphere, for the sclera does not cover the front of this sphere but encircles it. And this tunic is called the uvea because it is similar to the [skin of] a grape.⁑

5.7 ◉ Et in medio anterioris uvee est foramen rotundum perforatum usque ad eius concavum, et est oppositum extremitati concavitatis nervi super quam componitur oculus.

5.7 ◉ In the center of the anterior surface of the uvea is a round opening that passes into its hollow, and it lies opposite the end of the hollow of the nerve to which the eye is attached.⁑

5.8 ◉ Et cooperit istud foramen et omne anterius uvee in cuius circuitu circulatur consolidativa extrinsecus tunica fortis alba diafona, et dicitur cornea quoniam assimulatur cornu albo claro.

5.8 ◉ This opening and the entire front part of the uvea that the sclera encircles are covered over by a tough, white, transparent tunic called the cornea, because it is like white, clear horn.

5.9 ◉ Et in pectore concavi uvee est spera parva alba humida retentibilis humiditatis, et in ea est diafonitas non intensa valde, sed in ea est aliqua spissitudo. Et diafonitas eius as simulatur diafonitati glaciei, et ideo dicitur glacialis; et nominatur hoc nomine quoniam eius diafonitas assimulatur diafonitati glaciei. Et est composita super extremitatem concavitatis nervi, et in anteriori istius spere est compressio superficialis parva, et assimulatur compressioni superficiei lenticule. Superficies ergo anterioris eius est portio superficiei spere maioris superficie sperica continente duo eius foramina, et ista compressio est opposita foramini quod est in anteriori uvee, et situs eius ab eo est consimilis.

5.9 ◉ Toward the front of the uvea’s cavity lies a small, white, moist sphere that retains moisture, and, instead of a [perfectly] clear transparency it has some consistency. Its transparency, moreover, is like the transparency of ice, and therefore it is called the glacialis; and it takes this name because its transparency is like that of ice.⁑ It is attached to the endpoint of the hollow [optic] nerve, and in the anterior part of this sphere there is a slight flattening of the surface, and it is like the flattening of the surface of a lentil. Thus, its anterior surface is a section of the surface of a sphere that is larger than the spherical surface containing its two openings, and its flattened section faces the opening that lies at the front of the uvea, and it is equally situated with respect to it.⁑

5.10 ◉ Et iste humor dividitur in duas partes diverse diafonitatis: et altera illarum sequitur anterius eius, et altera sequitur eius posterius. Et diafonitas partis posterioris eius assimulatur diafonitati vitri quasi frustati, et ista pars dicitur humor vitreus. Et continet duas partes congregatas tela valde tenuis et quasi aranea quoniam assimulatur texture aranee.

5.10 ◉ This humor is divided into two parts of different transparency: one of them toward the front and the other toward the back. The transparency of its rear part is like that of ground glass,⁑ and this part is called the vitreous humor. The two parts together are surrounded by an extremely fine membrane called the aranea, because it is like a spider’s web in texture.

5.11 ◉ Et in pectore concavitatis uvee dicitur quod est foramen rotundum, et est super extremitatem concavitatis nervi. Et glacialis est composita in isto foramine, et rotunditas istius foraminis (et est extremitas nervi) continet medium spere glacialis; et consolidatur uvea cum glaciali ex circulo continenti istud foramen. Et dicitur quod crementum uvee est ex tunica intrinseca duarum tunicarum duorum nervorum obticorum et quod crementum cornee est ex tunica extrinseca duarum tunicarum istius nervi.

5.11 ◉ Furthermore, toward the front of the uvea’s hollow there is said to be a round opening, and it lies upon the endpoint of the hollow of the nerve. The glacialis is affixed in this opening, and the circumference of this opening (which is formed by the extremity of the nerve) encompasses the midpoint of the sphere of the glacialis; and the uvea is conjoined with the glacialis by the circle forming this opening. And it is said that the uvea arises from the inner tunic of the two tunics forming both hollow [optic] nerves and that the cornea arises from the outer tunic of the two tunics forming this nerve.⁑

5.12 ◉ Et implet concavitatem uvee humor albus tenuis clarus diafonus, et dicitur humor albugineus quoniam assimulatur albumini ovi in tenuitate, albedine, et diafonitate eius. Et ipse implet concavitatem uvee, et contingit anterius glacialis, et implet foramen quod est in anteriori uvee, et contingit concavum cornee.

5.12 ◉ A serous, white, clear, transparent humor fills the hollow of the uvea, and it is called the albugineous humor because it is like the white of an egg in its fluidity, whiteness, and transparency. And this humor fills the hollow of the uvea, and it is contiguous with the front surface of the glacialis, and it fills the opening in the front of the uvea, and it is contiguous with the concave surface of the cornea.⁑

5.13 ◉ Et spera glacialis est composita super concavitatem nervi, et sequitur concavitatem nervi humor vitreus. Erit ergo cornea, et humor albugineus, et humor glacialis et vitreus consequentes, et omnes iste tunice sunt diafone. Et foramen quod est in anteriori uvee est oppositum foramini concavitatis nervi. Erunt ergo inter superficiem cornee et anterius concavitatis nervi multe utilitates recte quoniam sunt diafona et contingentia se.

5.13 ◉ Now the sphere of the glacialis is affixed to the hollow of the [optic] nerve, and it is succeeded within that hollow by the vitreous humor. Thus, the cornea, the albugineous humor, the glacial humor, and the vitreous [humor] will lie one behind the other in that order, and all these tunics [and humors] are transparent. And the opening in the front of the uvea lies directly opposite the opening of the hollow of the [optic] nerve. Hence, between the surface of the cornea and the opening at the front of the hollow of the [optic] nerve there will be many straight-line connections since [all the intervening tunics and humors] are transparent and contiguous.⁑

5.14 ◉ Et dicitur quod spiritus visibilis emittitur ex anteriori cerebri et implet duas concavitates duorum nervorum primorum coniunctorum cum cerebro; et pervenit ad nervum communem, et implet concavitatem eius, et venit ad duos nervos secundos obticos. Et implet ipsos, et pervenit ad glacialem, et dat ei virtutem visibilem.

5.14 ◉ And it is said that visual spirit emanates from the front of the brain and fills the two hollows of the two nerves that are first joined with the brain; and this spirit extends to the common nerve, fills its hollow, and continues to the two secondary hollow nerves. It then fills them and continues to the glacialis to endow it with the power of seeing.⁑

5.15 ◉ Et inter circumferentiam glacialis coniunctam cum uvea et foramen quod est in concavo ossis ex quo exit nervus est spatium aliquantulum, et nervus extenditur in isto spatio ex fine foraminis usque ad circumferentiam glacialis secundum piramidalitatem et amplificationem. Et quantum elongabitur a foramine tanto magis amplificabitur quousque perveniat ad circumferentiam spere glacialis, et consolidatur cum circumferentia eius.

5.15 ◉ Between the circumference of the glacialis that is connected to the uvea and the opening in the hollow of the eye socket from which the nerve issues there is some space, and the nerve fills this space, from the very opening to the circumference of the glacialis as it expands and funnels outward. The farther from the opening it gets, the more it expands until it reaches the circumference of the sphere of the glacialis, and it is affixed to its circumference.

5.16 ◉ Et corpus consolidative continet istam partem piramidalem nervi, et continet speram uveam, et spera uvea antecedit medium consolidative ad partem manifestam oculi. Et corpus consolidative est consolidatum cum spera uvea et cum extremitate piramidali nervi et custodiens situm eius. Cum ergo movetur oculus, movebitur secundum totum. Et sic declinabitur nervus super quem componitur oculus apud motum eius, et erit declinatio apud foramen quod est in concavitate ossis, quoniam concavitas ossis continet totum oculum, et oculus movetur secundum totum in ista concavitate.

5.16 ◉ The body of the sclera encompasses this expanded portion of the nerve, and it encompasses the uveal sphere, but the uveal sphere lies in front of the midpoint of the sclera toward the [front] surface of the eye. The body of the sclera is joined with the uveal sphere as well as with the endpoint of the expanding nerve and keeps it fixed in place. Hence, when the eye moves, it will move as a whole. And thus the nerve to which the eye is affixed will follow its movement and will flex at the opening in the hollow of the eye socket, because the hollow of the eye socket contains the entire eyeball, and the eyeball moves as a whole within this hollow.⁑

5.17 ◉ Et consolidativa consolidatur cum eo quod est in anteriori eius ex nervo et ex tunicis residuis, et est custodiens semper situm eius. Declinatio ergo nervi apud motum oculi non est nisi a posteriori totius oculi; est ergo apud foramen quod est in concavitate ossis. Et similiter quando oculus fuerit quiescens et nervus fuerit declinans, non erit nisi apud foramen quod est in concavitate ossis. Nam non alteratur situs partium totius oculi adinvicem nec apud motum nec apud quietem. Declinatio ergo nervi super quem componitur oculus non est nisi apud foramen quod est in concavitate ossis sive moveatur oculus sive quiescat.

5.17 ◉ The sclera is also connected to the part of the nerve that lies toward the front [of the eye] as well as to the rest of the tunics, so it holds them [all] firmly in place. Thus, the flexing of the nerve with the motion of the eye occurs only at the back of the eye; so it happens at the opening in the hollow of the eye socket. Likewise, when the eye is still and the nerve is flexed, that flexing will occur only at the opening in the hollow of the eye socket. For the parts of the whole eye do not shift with respect to each other either when it is in motion or when it is still. Thus, the flexing of the nerve to which the eye is attached only happens at the opening in the hollow of the eye socket, whether the eye is moving or is still.

5.18 ◉ Superficies autem manifesta cornee est superficies sperica et cum hoc est continuata cum superficie totius oculi et cum toto oculo. Et totus oculus est maior spera uvea que est quidam eius. Superficies autem manifesta cornee cum superficie totius oculi est et maior superficie spere uvee. Semidyameter ergo eius est maior semidyametro uvee.

5.18 ◉ The outer surface of the cornea is spherical and is therefore continuous with the surface of the entire eye and with the whole eyeball.⁑ The eye as a whole forms a sphere larger than the uveal sphere, which is one of its parts. However, the outer surface of the cornea is continuous with the surface of the entire eye, which is larger than the surface of the uveal sphere. Its radius is therefore larger than the radius of the uvea.

5.19 ◉ Et superficies intrinseca cornee superposita foramini uvee est superficies concava sperica equidistans superficiei manifeste ipsius, quoniam iste locus est equalis spissitudinis. Centrum ergo istius superficiei concave est idem cum centro superficiei manifeste convexe, et ista superficies concava secat superficiem spere uvee super circumferentiam foraminis. Centrum ergo eius est remotius in profundo quam centrum uvee, quoniam hoc est certum in proprietatibus sperarum.

5.19 ◉ The inner surface of the cornea that is positioned over the opening in the uvea is a concave spherical surface parallel to its outer surface, for this section of the eye is of equal thickness. The center of this concave surface is therefore the same as the center of the outer, convex surface, and this concave surface intersects the surface of the uveal sphere at the circumference of its opening. Therefore its center lies deeper in the eye than the center of the uvea, for this follows inexorably from the properties of [intersecting] spheres.

5.20 ◉ Et etiam quia spera uvee non est in medio consolidative et est antecedens ad partem superficiei manifesti oculi, et superficies manifesta oculi est ex spera maiori spera uvee, erit centrum superficei manifeste remotius in profundo centro uvee.

5.20 ◉ In addition, since the uveal sphere is not concentric with the sclera but lies in front toward the outer surface of the eye, and since the outer surface of the eye forms part of a sphere that is larger than the uvea, the center of the outer surface [of the eye] will lie deeper in the eye than the center of the uvea.

5.21 ◉ Et linea recta que continuat duo centra—scilicet centrum superficiei cornee et centrum uvee—quando extrahitur recte, pervenit ad centrum foraminis quod est in anteriori uvee et ad duo media duarum superficierum cornee equidistantium. Superficies enim concava cornee et superficies convexa uvee sunt superficies sperice secantes se. Nam linea que continuat centra eorum transit per centrum circuli sectionis, et erit perpendicularis super superficiem eius, nam linea que exit a centro circuli et est perpendicularis super superficiem eius transit per centra duarum sperarum.

5.21 ◉ Moreover, when it is extended, the straight line that connects the two centerpoints—i.e., the center of the cornea’s [outer or inner] surface and the center of the uvea—passes through the center of the opening at the front of the uvea as well as through the midpoints of the two parallel corneal surfaces. For the concave surface of the cornea and the convex surface of the uvea are intersecting spherical surfaces. Now the line that joins their centers passes through the center of the circle of intersection, and it will be perpendicular to its surface, for a line dropped to the center of [such a] circle and perpendicular to its surface passes through the centers of the two [intersecting] spheres.⁑

5.22 ◉ Et superficies concava cornee contingit superficiem humoris albuginei que est in anteriori foraminis uvee, et superponitur ipsi. Superficies ergo humoris albuginei etiam est superficies sperica cuius centrum est centrum superficiei ei superposite. Superficies ergo manifesta cornee, et superficies intrinseca ipsius, et superficies humoris albuginei que contingit concavum cornee sunt superficies sperice equidistantes. Et centrum earum est unum punctum commune, et est remotius in profundo centro uvee.

5.22 ◉ The concave surface of the cornea is contiguous with the surface of the albugineous humor at the front of the uveal opening, and it covers it. Thus, the surface of the albugineous humor is also a spherical surface whose center coincides with the center of the surface that covers it. So the outer surface of the cornea, as well as its inner surface, and the surface of the albugineous humor contiguous with the concave surface of the cornea are parallel spherical surfaces. Moreover, their centers form a common point that is deeper inside [the eye] than the uvea’s center.

5.23 ◉ Et linea que transit per centrum uvee, et per centrum cornee, et per centrum foraminis quod est in anteriori uvee, quando extenditur recte, transibit per medium concavitatis nervi super quem componitur oculus, quoniam foramen quod est in anteriori uvee est oppositum foramini quod est in pectore uvee quod est extremitas concavitatis nervi.

5.23 ◉ When the line passing through the center of the uvea, the center of the cornea, and the center of the opening at the front of the uvea is extended rectilinearly, it will pass through the middle of the hollow of the nerve to which the eye is attached, for the opening at the front of the uvea lies opposite the opening within the body of the uveal sphere that forms the extremity of the hollow of the nerve [where its expanded end attaches to the uveal sphere].

5.24 ◉ Et superficies anterioris glacialis etiam est sperica superficies, et ipsa secat speram uvee; centrum ergo eius est remotius in profundo centro uvee. Et linea recta que continuat duo centra eorum transit per centrum circuli sectionis, et etiam est perpendicularis super ipsum. Et circulus sectionis inter superficiem anterioris glacialis et superficiem spere uvee est aut circulus distinguens finem consolidationis inter glacialem et uveam aut equidistans ei. Quoniam superficies que est in anteriori glacialis est opposita foramini quod est in anteriori uvee, et situs eius est consimilis ex eo. Finis ergo istius superficieiet est circulus sectionis inter duas superficies glacialis—aut est ipse circulus consolidationis aut equidistans ei.

5.24 ◉ The anterior surface of the glacialis is also a spherical surface, and it intersects the uveal sphere; so its center lies deeper [in the eye] than the center of the uvea. And the straight line connecting these two centerpoints passes through the center of the circle of intersection, so it is also perpendicular to it. But the circle of intersection between the surface at the front of the glacialis and the surface of the uveal sphere forms either the circle defining the boundary between glacialis and uvea or a circle parallel to that one. For the surface at the front of the glacialis is opposite the opening at the front of the uvea, and it is uniformly positioned with respect to it. Thus, the boundary of this surface—which is the circle of intersection between the two surfaces of the glacialis—is either the circle of attachment itself or a circle parallel to that circle.⁑

5.25 ◉ Si ergo circulus sectionis inter duas superficies glacialis fuerit circulus consolidationis, iste circulus ergo est circulus sectionis inter superficiem anterioris glacialis et inter superficiem uvee. Et si circulus sectionis inter duas superficies glacialis fuerit equidistans circulo consolidationis spere glacialis cum uvea (hoc quidem accidit si fuerit consolidatio in parte posteriori glacialis), erit superficies anterioris glacialis, quando fuerit ymaginata extensa super illud super quod est ex sua spera, secans speram uvee super circulum equidistantem isti circulo—scilicet circulo sectionis inter duas superficies glacialis—propter consimilitudinem situs istius circuli ad circumferentiam spere uvee. Et iste circulus est equidistans circulo consolidationis. Erit ergo circulus sectionis inter superficiem anterioris glacialis et inter speram uveam aut ipse circulus consolidationis aut ei equidistans. Si ergo iste circulus fuerit ipse circulus consolidationis, linea recta que transit per centrum anterioris glacialis et per centrum uvee transibit per centrum istius circuli, et erit perpendicularis super ipsum, quoniam iste circulus erit circulus sectionis inter duas superficies spericas. Et si iste circulus fuerit equidistans circulo consolidationis et est equidistans circulo sectionis inter duas superficies glacialis, est ergo cum circulo sectionis inter duas superficies glacialis in superficie sperica—et est superficies anterioris glacialis—et est equidistans ei. Linea ergo que transit per centrum uvee et per centrum superficiei antecedentis glacialis transit per centrum circuli consolidationis super omnes dispositiones. Et erit perpendicularis super ipsum sive sit circulus consolidationis ipse circulus sectionis inter superficiem anterioris glacialis et inter speram uvee aut sit equidistans isti circulo.

5.25 ◉ Accordingly, if the circle of intersection between the two surfaces of the glacialis is the circle of attachment itself, then this circle forms the circle of intersection between the anterior surface of the glacialis and the [inner] surface of the uvea. But if the circle of intersection between the two surfaces of the glacialis is parallel to the circle of attachment connecting the sphere of the glacialis and the uvea (which is certainly the case if the attachment occurs at the rear portion of the glacialis), then, if it is imagined to enlarge beyond its present spherical limits, the anterior surface of the glacialis will intersect the uveal sphere to form a circle parallel to that circle—i.e., the circle of intersection between the two surfaces of the glacialis—on account of the uniform placement of this circle with respect to the circumference of the uveal sphere. And this circle is parallel to the circle of attachment. Hence, the circle of intersection between the front surface of the glacialis and the uveal sphere will be either the circle of attachment itself or a circle parallel to it. Accordingly, if this circle is the circle of attachment itself, then the straight line passing through the center of the anterior [surface] of the glacialis and the center of the uvea will pass through the center of this circle and will be perpendicular to it, because this circle will be the circle of intersection between two spherical surfaces. But if this circle is parallel to the circle of attachment and is parallel to the circle of intersection between the two surfaces of the glacialis, then it lies on the same spherical surface as the circle of intersection between the two surfaces of the glacialis—i.e., the anterior surface of the glacialis—and it is parallel to it. Consequently, the line that passes through the center of the uveal sphere and the center of the surface at the front of the glacialis passes through the center of the circle of attachment in all situations. And it will be perpendicular to that circle whether the circle of attachment is the [actual] circle of intersection between the front surface of the glacialis and the uveal sphere or whether it is parallel to that circle.⁑

5.26 ◉ Et etiam superficies anterioris glacialis et superficies residui glacialis sunt due superficies sperice secantes se. Centrum ergo superficiei antecedentis est remotius in profundo centro superficiei posterioris; et linea recta que continuat ista duo centra transit per centrum circuli sectionis, et erit perpendicularis super ipsum. Et iam declaratum est quod transit per centrum circuli consolidationis et est perpendicularis super ipsum, nam iste circulus aut est circulus consolidationis aut equidistans ei. Linea ergo que transit per centrum uvee, et per centrum anterioris glacialis, et per centrum circuli consolidationis (et est perpendicularis super istum circulum) transit per centrum residui glacialis.

5.26 ◉ Also, the anterior surface of the glacialis and the surface of the rest of the glacialis are two intersecting spherical surfaces. Thus, the center of the front surface lies deeper [within the eye] than the center of the rear surface;⁑ and the straight line connecting these two centers passes through the center of the circle of intersection, and it will be perpendicular to it. And it has already been shown that this line passes through the circle of attachment and is perpendicular to it, for this circle [of intersection] is either the circle of attachment itself or is parallel to it. Thus, the line passing through the center of the uvea, as well as through the center of the anterior [surface] of the glacialis and the center of the circle of attachment (this line being perpendicular to this circle) passes through the center of the remaining portion of the glacialis.

5.27 ◉ Et cum ista linea transit per centrum residui glacialis et per centrum circuli consolidationis, et est erecta super superficiem circuli consolidationis secundum angulos rectos, extenditur ergo in medio concavitatis nervi super quem componitur oculus, quoniam circulus consolidationis est extremitas concavitatis nervi.

5.27 ◉ And since this line passes through the center of the remaining portion of the glacialis as well as through the center of the circle of attachment, and since it stands at right angles to the surface of the circle of attachment, then it extends through the middle of the hollow of the nerve to which the eye is attached, because the circle of attachment coincides with the extremity of the hollow of the nerve.

5.28 ◉ Et iam declaratum est quod linea transiens per centrum uvee, et per centrum cornee, et per centrum foraminis quod est in exteriori sive anteriori uvee extenditur in medio concavitatis nervi. Ista ergo linea que transit per duo centra superficiei glacialis et per centrum uvee est ipsa linea que transit per centrum cornee, et per centrum uvee, et per centrum foraminis quod est in anteriori uvee. Ista ergo linea transit per centrum cornee, et per centrum uvee, et per duo centra superficiei glacialis, et per centrum foraminis quod est in anteriori uvee, et per centrum circuli consolidationis. Et transit per duo media tunicarum omnium oppositarum foramini uvee, et est perpendicularis super superficies omnium tunicarum oppositarum foramini uvee. Et est perpendicularis super superficiem foraminis uvee et perpendicularis super superficiem circuli consolidationis, et extenditur in medio concavitatis nervi super quem componitur oculus.

5.28 ◉ And it has already been shown that the line passing through the center of the uvea, the center of the cornea, and the center of the opening which is at the outer or front [surface] of the uvea extends through the middle of the hollow of the nerve.⁑ The line, therefore, that passes through the two centers of the surface[s] of the glacialis, as well as through the center of the uvea, is the very line that passes through the center of the cornea, the center of the uvea, and the center of the opening at the front of the uvea. So this line passes through the center of the cornea, the center of the uvea, the two centers of the surface[s] of the glacialis, the center of the opening at the front of the uvea, and the center of the circle of attachment. It also passes through the two centers of all the tunics facing the opening in the uvea,⁑ and it is perpendicular to the surfaces of all the tunics facing the uvea’s opening. It is perpendicular as well to the surface of the uvea’s opening and to the circle of attachment, and it extends through the middle of the hollow of the nerve to which the eye is attached.

5.29 ◉ Et cum declaratum sit quod centrum cornee et centrum superficiei anterioris glacialis ambo sunt super istam lineam et ambo sunt remotiora in profundo centro uvee, melius est ut centrum superficiei anterioris glacialis sit ipsum centrum cornee, ita quod centra omnium superficierum oppositarum foramini uvee sint unum punctum commune. Et sic erunt omnes linee exeuntes a centro ad superficiem oculi perpendiculares super omnes superficies oppositas foramini. Et cum hoc posterius declarabitur apud nostrum sermonem in qualitate visionis quod centrum superficiei cornee et centrum superficiei anterioris glacialis est unum centrum commune. Superficies ergo tunicarum visus oppositarum foramini uvee sunt superficies sperice quarum centrum est unum punctum commune.

5.29 ◉ And since it has been shown that both the center of the cornea and the center of the anterior surface of the glacialis lie upon this line and that both lie deeper [in the eye] than the center of the uvea, it is perfectly appropriate for the center of the anterior surface of the glacialis to be the same as the center of the cornea, so that the centers of all the surfaces facing the opening in the uvea form a single, common point. Hence, all the lines projected from that centerpoint to the surface of the eye will be perpendicular to all the surfaces facing the [uveal] opening.⁑ Accordingly, we will later show in our discussion of how vision takes place that the center of the corneal surface and the center of the anterior surface of the glacialis form a single, common center. Thus, the surfaces of the tunics of the eye that face the opening in the uvea form spherical surfaces that share a single, common centerpoint.⁑

5.30 ◉ Et etiam quia istud centrum est centrum superficiei manifeste oculi continuate cum superficie continente totum oculum (et totus oculus est rotundus nisi quantum deficit de completione spere pinguedinis consolidative a parte anteriori ipsius oculi, et iste defectus non operatur diversitatem in motu oculi quoniam non tangit concavum ossis) istud ergo centrum erit centrum totius oculi. Ergo est intra totum oculum. Centrum ergo superficierum tunicarum visus oppositarum foramini uvee est intra totum oculum.

5.30 ◉ In addition, because this centerpoint forms the center of the outer surface of the eye that is continuous with the surface enclosing the whole eye (and the entire eye is round save for the bit that the sphere of fat forming the sclera lacks at the front of the eye, and this shortfall makes no difference in the eye’s motion since it is not in contact with the cavity in the eye socket), this centerpoint will be the centerpoint for the entire eye. Hence, it lies inside the eye as a whole. The centerpoint of the surfaces of the tunics of the eye facing the uveal opening therefore lies inside the eye as a whole.

5.31 ◉ Cum ergo movetur oculus, non mutabitur punctus oculi quod est centrum superficierum tunicarum visus, nec mutabitur situs eius ab istis superficiebus. Sed est custodiens situm eius, nam oculus, quando movetur, non movetur nisi secundum totum, et situs partium totius adinvicem non mutatur apud motum. Et istud centrum est intra; situs ergo eius non mutatur apud suum motum. Et similiter situs tunicarum visus non mutatur apud totum oculum—id est apud motum ipsius visus—situs ergo istius centri apud superficies tunicarum visus non mutatur nec in motu nec in quiete.

5.31 ◉ When the eyeball moves, then, the point within the eye that forms the center of the surfaces of the tunics of the eye will not shift [in relation to the eye socket], nor will it shift in relation to those surfaces. On the contrary, it stays fixed, for when the eye moves it moves only as a whole, and the parts of that whole do not move in relation to one another when it moves. But this centerpoint lies within [the eye as a whole], so it does not move with the motion of that whole. Likewise, the tunics of the eye do not move with the motion of the eye as a whole—i.e., with the motion of the eye itself—so this centerpoint does not move in relation to the surfaces of the tunics, whether [the eye is] in motion or at rest.

5.32 ◉ Et iam declaratum est quod declinatio nervi apud motum visus et apud quietem non est nisi apud foramen quod est in concavitate ossis, quoniam non est nisi a posteriori totius oculi. Declinatio vero nervi apud motum visus et quietem eius non est nisi a posteriori centri eius.

5.32 ◉ And it has already been shown that the flexing of the nerve when the eye moves or when it is immobile occurs only at the opening in the cavity of the eye socket, because it only takes place at the very back of the eye.⁑ It follows that the flexing of the nerve when the eye is moving or at rest only takes place behind the eye’s centerpoint.

5.33 ◉ Et etiam non mutatur situs partium totius oculi adinvicem nec in motu nec in quiete. Situs ergo centrorum tunicarum oculi apud totum oculum non mutatur nec in motu visus nec in quiete. Linea ergo recta transiens per centrum non mutat suum situm apud totum oculum nec apud partes eius, sed nec in motu nec in quiete. Et cum situs istius linee non mutetur apud totum oculum nec apud partes eius, situs ergo istius linee non mutatur apud superficiem circuli consolidationis nec apud suam circumferentiam. Et iste circulus est extremitas concavitatis nervi. Situs ergo superficiei eius a superficie concavitatis nervi est situs consimilis; et declinatio partis piramidalis nervi super superficiem istius circuli est declinatio consimilis, quoniam situs glacialis ab isto nervo est situs consimilis.

5.33 ◉ Nor do the parts of the eye move with respect to each other whether [the eye is] in motion or at rest. Thus, the centerpoints of the eye’s tunics do not move with respect to the eye as a whole, whether the eye is in motion or at rest. Accordingly, the straight line passing through the centerpoint does not move with respect to the eye as a whole or to its parts, no matter whether [the eye is] in motion or at rest. And since this line moves with respect neither to the eye as a whole nor to its parts, then this line does not move with respect to the surface of the circle of attachment or its circumference. But this circle forms the extremity of the hollow of the [optic] nerve. Thus, its surface and the surface of the nerve’s hollow have the same orientation; and the inclination of the funnel-shaped portion of the nerve to the surface of this circle is constant, because the glacialis maintains a constant orientation with respect to this nerve.

5.34 ◉ Et cum situs partium oculi non mutatur adinvicem, superficies ergo concavitatis nervi a loco circumferentie circuli consolidationis usque ad locum declinationis nervi qui est pars piramidalis non mutat situm eius apud totum oculum nec apud circulum consolidationis.

5.34 ◉ Since the parts of the eye do not move with respect to one another, the surface of the [optic] nerve’s hollow, from the circumference of the circle of attachment to the place where the funnel-shaped part of the nerve begins to flare outward, moves with respect neither to the eye as a whole nor to the circle of attachment.

5.35 ◉ Et iam declaratum est quod situs linee que transit per centra non mutatur apud circulum consolidationis et quod ipsa extenditur in medio concavitatis nervi. Et cum situs istius linee non mutatur apud circulum consolidationis, nec superficies concavitatis nervi que est a loco circumferentie circuli consolidationis usque ad locum declinationis mutat suum situm apud circulum consolidationis, ista ergo linea non mutat suum situm apud concavitatem nervi quousque pervenit ad locum declinationis. Linea ergo que transit per centrum tunicarum visus transit per centrum circuli consolidationis, et erit erecta super ipsum secundum angulos rectos, et extenditur in medio concavitatis nervi piramidalis quousque perveniat ad locum declinationis nervi. Et erit situs suus semper a superficie concavitatis nervi que est intra totum oculum, et ab omnibus partibus oculi, et ab omnibus superficiebus tunicarum visus idem situs, et non mutatur nec in motu visus aut in quiete eius.

5.35 ◉ Furthermore, it has already been shown that the line passing through the centers [of the ocular components] does not move with respect to the circle of attachment and that this line extends through the middle of the [optic] nerve’s hollow.⁑ But if this line does not move with respect to the circle of attachment, and if the surface of the nerve’s hollow from the circumference of the circle of attachment to the place [in the eye socket] where it flexes does not move with respect to the circle of attachment, then this line does not move with respect to the hollow of the nerve up to the point where it flexes. Thus, the line that passes through the center of the tunics of the eye passes through the center of the circle of attachment, and it will stand at right angles to it, and it extends through the middle of the hollow of the funnel-shaped portion of the nerve up to the point where the nerve flexes. It will always maintain a constant position with respect to the surface of the nerve’s hollow within the eye, as well as [with respect] to all the parts of the eye and all the surfaces of the tunics of the eye, and it does not change that position whether the eye is moving or at rest.

5.36 ◉ Isti ergo sunt situs tunicarum visus et situs centrorum earum et situs linee recte transeuntis per centra earum.

5.36 ◉ These, therefore, are the dispositions of the tunics of the eye, the dispositions of their centerpoints, and the disposition of the straight line passing through their centerpoints.

5.37 ◉ Oculi autem ambo sunt consimiles in omnibus suis dispositionibus et in suis tunicis, et in figuris suarum tunicarum, et in situ cuiuslibet tunice respectu totius oculi. Et cum ita est, situs cuiuslibet centrorum quorum distinctio declarata fuit apud totum oculum et apud partes eius est sicut situs centri respondentis illi centro in alio oculo apud totum illum oculum et apud partes eius. Et cum situs centrorum in utroque oculo est similis situi suorum respondentium in oculo reliquo, erit situs linee transeuntis per centra in uno oculorum apud totum oculum, et apud partes eius, et apud suas tunicas similis situi linee transeuntis per centra alterius oculi apud totum oculum, et apud partes eius, et apud suas tunicas. Situs ergo linearum transeuntium duarum per centra tunicarum visus ab utroque oculorum est situs consimilis in omnibus suis dispositionibus.

5.37 ◉ Moreover, both eyes are similar in all respects, with regard to their tunics, as well as to the shape of their tunics and the situation of each of the tunics with respect to the eye as a whole. And given this fact, the location of each of the previously discussed centerpoints with respect to the whole of one eye as well as to its parts corresponds to that of the centerpoints of the other eye as a whole as well as to its parts. And since the location of the centerpoints in either eye corresponds to the location of the centerpoints in its mate, the line passing through the centerpoints of one eye will be similarly situated in respect to the eye as a whole, its parts, and its tunics as the line passing through the centerpoints of the other eye in respect to that eye as a whole, its parts, and its tunics. Thus, the two lines passing through the centerpoints of the tunics of both eyes are similarly situated in all respects.

5.38 ◉ Et utraque consolidativarum consolidatur cum eis, cum ex eis exeunt duo lacerti parvuli, et unus eorum est in parte lacrimarum oculi et alius in parte posteriori. Et continent utrumque oculorum palpebre et cilia.

5.38 ◉ Each of the scleras is affixed with these components [into the eye-sockets], for two small muscles grow out of them, one toward the side of the tear ducts, the other toward the back edge. And lids and eyelashes cover both eyes.

5.39 ◉ Hoc ergo quod declaravimus est dispositio compositionis oculi et forma eius et forma suarum tunicarum. Et omne quod diximus ex tunicis oculi et compositione earum iam declaratum est ab anatomicis in libris anatomie, et ista est forma oculi.

5.39 ◉ What we have thus shown is how the eye is composed, its [overall] structure, and the structure of its [component] tunics. And everything we have said about the tunics of the eye and their structure has already been shown by anatomists in books on anatomy,⁑ and this is the way the eye is formed.

◉[Capitulum 7]

◉[Chapter 7]

6.1 ◉ Iam declaratum est superius quod ex quolibet corpore illuminato cum quolibet lumine exit lux ad quamlibet partem oppositam ei. Cum ergo visus opponitur alicui rei vise et fuerit res illa illuminata cum quolibet lumine, ex lumine rei vise veniet lumen ad superficiem visus. Et declaratum fuit quod ex proprietate lucis est operari in visum et quod natura visus est pati ex luce. Dignum ergo est ut non sentiat visus lumen rei vise nisi ex lumine veniente ex ea ad visum.

6.1 ◉ It has already been shown above that light emanates in every [possible] direction from any luminous body, however it is illuminated.⁑ Thus, when the eye faces any visible object that shines with some sort of illumination, light from that visible object will shine on the eye’s surface. And it was shown that it is a property of light to affect sight, whereas it is in the nature of sight to be affected by light. It is therefore fitting that sight sense the luminosity of a visible object only through the light that shines from it upon the eye.

6.2 ◉ Et declaratum fuit iam quod forma coloris cuiuslibet corporis colorati et illuminati cum quolibet lumine associatur semper lumen veniens ab illo corpore ad quamlibet partem oppositam illi corpori, et erit lumen et forma coloris semper similis. Ergo cum lumine veniente ad visum ex lumine corporis visi erit semper forma coloris corporis visi, et cum lumen et color venient simul ad superficiem visus, et visus sentit colorem qui est in re visa ex lumine veniente ei ex re visa, dignius est ut non sit sensus visus coloris rei vise nisi ex forma coloris venientis ad ipsum visum cum lumine.

6.2 ◉ It was also shown earlier that the form of the color of any tinted body that shines with any sort of illumination is always mingled with the light shining in every direction from that body, and light and the form of color will always correspond with one another.⁑ Therefore, since the form of the color of the visible object will always coexist with the light shining from the visible object to the eye, and since light and color will reach the surface of the eye together, and since sight senses the color that is in the visible object by means of the light shining upon it from the visible object, it is quite fitting that sight sense the color of the visible object only from the form of [that] color reaching the eye along with the light [shining from the object].

6.3 ◉ Et etiam forma coloris semper est admixta cum forma luminis, et non est distincta ab eo. Visus ergo non sentit lumen nisi admixtum cum colore. Dignius ergo est ut non sit sensus visus coloris rei vise et luminis quod est in ea nisi ex forma admixta ex lumine et colore veniente ad ipsum ex superficie rei vise.

6.3 ◉ Also, the form of color is always mingled with the form of light and is not separable from it. So sight senses light only when it is mingled with color.⁑ It is thus quite fitting that sight sense the color and light that are in the visible object only through a form that is composed of both the light and color shining upon it from the surface of the visible object.

6.4 ◉ Et etiam tunice visus que situantur ad medium anterioris visus sunt diafone contingentes se, et prima illarum, scilicet cornea, tangit aerem in quo primo venit forma. Et ex pro prietatibus lucis est pertransire in quodlibet corpus diafonum, et similiter est proprietas forme coloris que associatur lumini pertransire in corpus diafonum. Et ideo extenditur in aere diafono sicut extenditur lumen. Et ex natura corporum diafonorum est recipere formas lucis et colorum et redere ipsas partibus oppositis illi. Forma ergo veniens ex re visa ad superficiem visus transibit per diafonitatem tunicarum visus ex foramine quod est in anteriori uvee. Perveniet ergo ad humorem glacialem et pertransibit in eo etiam secundum suam diafonitatem. Dignius ergo est ut tunice visus non fuerint diafone nisi ut pertranseant in eis forme lucis et colorum venientium ad ipsum.

6.4 ◉ In addition, the tunics of the eye that are centered on the front of the eye are contiguous and transparent, and the first of these, i.e., the cornea, is in contact with the air that initially transmits the form. But it is among the properties of light to pass through any transparent body, and it is likewise a property of the form of color that it mingles with light in order to pass through a[ny] transparent body. Therefore, it extends through the transparent air in the same way as light. And it is in the nature of transparent bodies to receive the forms of light and colors and to transmit them in facing directions.⁑ Hence, the form that comes from the visible object to the surface of the eye will pass through the transparency of the tunics of the eye from the opening that is at the front of the uvea. It will therefore reach the glacial humor and will also pass through it on account of its transparency. It is thus quite fitting for the tunics of the eye to be transparent for the sole purpose of letting the forms of light and colors that reach the eye pass through.

6.5 ◉ Aggregemus ergo modo quod componitur ex omnibus istis.

6.5 ◉ At this juncture, then, let us summarize all of these points.

6.6 ◉ Et dicemus quod visus sentit lumen et colores que sunt in superficie rei vise, et pertranseunt per diafonitatem tunicarum visus. Et hoc est illud in quo quiescebat opinio naturalium in qualitate visionis.

6.6 ◉ And we will say that sight senses the light and colors that are in the surface of the visible object and that they pass through the transparency of the tunics of the eye. This is by now the accepted opinion of natural philosophers about how vision occurs.⁑

6.7 ◉ Dicemus ergo modo quod qualitas visionis non asseritur esse huiusmodi tantum, quoniam iste modus destruitur nisi addatur ei aliud, quoniam forma lucis et coloris cuiuslibet corporis colorati et illuminati extenditur in aere diafono continuato cum eo ad omnes partes oppositas. Visus autem opponitur in eodem tempore rebus multis visis diversi coloris, et inter quamlibet earum et visum sunt in aere linee recte continuato medio inter eas. Et cum forme lucis et coloris que sunt in re visa opposita visui venient ad superficiem visus, forme lucis et coloris cuiuslibet rerum visibilium oppositarum visui in eodem tempore veniunt in illo tempore ad superficiem visus. Et cum forme extenduntur ex re visa ad quamlibet partem oppositam et non perveniunt ad visum nisi propter oppositionem, forma que pervenit ex re visa ad visum pervenit ad totam superficiem visus. Et cum ita est, quando visus opponetur alicui superficiei rei vise, et pervenerit forma coloris eius et lucis ad superficiem visus, et viderit in illo tempore aspiciens alia visibilia diversi coloris opposita visui, tunc forma lucis et coloris cuiuslibet illorum visibilium veniet ad superficiem visus. Et erit forma omnium illorum visibilium perveniens ad totam superficiem visus. Perveniet ergo ad totam superficiem visus et in tota multa lumina diversa et multi colores diversi, et quodlibet illorum implet superficiem visus. Pervenit ergo in superficie visus forma admixta ex coloribus diversis et luminibus diversis.

6.7 ◉ We will now say that this alone does not suffice to describe the way vision occurs, for, without some additional qualification, this explanation does not stand, for the form of light and color of any colored and illuminated body extends in all directions through the transparent air that is contiguous with it. However, the eye faces several visible objects of different colors at the same time, and between each of them and the eye there are direct lines through the continuum of air that links them. And since the forms of light and color that are in a visible object facing the eye will reach its surface, the forms of the light and color belonging to any of the visible objects facing the eye at the same time reach the surface of the eye at the same time. And since the forms extend from the visible object to any facing point and reach the eye only when it faces [that object], the form that comes from the visible object to the eye reaches the entire surface of the eye. And since this is the case, when the eye faces any surface of a visible object, if the form of its color and light reaches the eye’s surface, and if at that time the observer sees other visible objects of a different color that face the eye, then the form of the light and color of any of those visible objects will reach the eye’s surface. And the form of all of those visible objects will reach the entire surface of the eye. On the whole, then, several different lights and several different colors will reach the entire surface of the eye, and each of them fills the surface of the eye. So a form composed of various colors and lights reaches the surface of the eye.

6.8 ◉ Si ergo visus senserit illam formam admixtam, sentiet colorem diversum a colore cuiuslibet illarum rerum, et non distinguentur ab eo visibilia. Et si senserit unam illarum rerum visibilium et non senserit residuas, comprehendet unam rem visibilem et non alias. Sed ipse comprehendit omnia illa visibilia in eodem tempore, et comprehendit ipsa distincta.

6.8 ◉ If sight were then to sense that composite form, it would sense a color different from the color of any one of the objects, and it would not distinguish [any of the component] visible objects through it. Yet, if it were to sense one of those visible objects and were not to sense the rest, it would discern one visible object but not the others. But it discerns all of those visible objects at the same time, and it discerns them [all] distinctly.

6.9 ◉ Et si non senserit unam illarum formarum, nichil sentiet ex ipsis visibilibus oppositis illi. Sed ipse sentit omnia.

6.9 ◉ On the other hand, if it were unable to sense [any] one of those forms, it would sense none of the visible objects facing it. But it senses them all.

6.10 ◉ Et iterum erunt in eodem viso diversi colores et lineares secundum ordinem, et a qualibet parte eius exit lumen et color secundum omnes lineas rectas que extenduntur in aere continuo. Cum ergo fuerint partes unius rei vise diversi coloris, veniet ad totam superficiem visus ex unoquoque illorum forma coloris et lucis; et sic admiscebuntur colores illarum partium in superficie visus, quare visus comprehendet ipsos aut admixtos aut nichil comprehendet ex eis. Si vero comprehendit eos ad mixtos, non distinguentur nec ordinabuntur ab eo partes sive colores partium. Et si nichil comprehendit ex illis formis, nichil comprehendit ex partibus; et si nichil ex partibus, nichil comprehendet ex re visa. Sed visus comprehendit rem visam illuminatam oppositam sibi, et comprehendit partes eius diversi coloris ordinatas et distinctas.

6.10 ◉ Furthermore, in the same visible object there will be different colors and designs according to some arrangement, and from any spot on that object light and color emanate along every straight line that extends [from it] through the continuous air. Therefore, since the parts of a single visible object have different colors, from any one of those spots the form of color and light will reach the entire surface of the eye; and thus the colors of those parts will mingle on the eye’s surface, whence sight will either perceive them mingled together or will perceive none of them. Yet if it perceives them mingled together, neither the parts themselves nor their colors will be discerned or perceived according to their proper arrangement. And if it fails to perceive any of their forms, it fails to perceive any of their parts; and if it fails to grasp any of their parts, it will fail to perceive the visible object [as a whole]. But sight does perceive any illuminated visible object facing it, and it perceives the parts of it that are of different colors distinctly and according to their proper arrangement.

6.11 ◉ Et cum ita est, constat quoniam aut qualitas visionis erit alio modo aut erit iste modus pars modi videndi. Debemus ergo considerare utrum iste modus possit convenire conditionibus per quas distinguuntur colores rerum visibilium, et ordinantur partes eorum apud visum et erunt convenientes ad esse.

6.11 ◉ This being the case, it follows either that vision will take place in some other way or that this account will only be part of the story. Therefore, we ought to consider whether this account can be suited to the circumstances under which the colors of visible objects are distinguished, and the parts of those objects are perceived by sight according to their proper arrangement, so they will correspond to reality.

6.12 ◉ Dicemus ergo quod quando visus fuerit oppositus alicui rei visibili, veniet ex quolibet puncto superficiei rei vise forma et coloris et lucis que sunt in ea ad totam superficiem visus. Et ex quolibet puncto cuiuslibet rerum visibilium oppositarum visui in illa dispositione etiam venient forme coloris et lucis que sunt in illo ad totam superficiem visus. Si ergo visus senserit ex tota eius superficie formas coloris et lucis que veniunt ex aliquo puncto superficiei rei vise, sentiet ex tota eius superficie formam cuiuslibet puncti superficiei illius rei vise et formam cuiuslibet puncti superficierum omnium rerum visibilium oppositarum illi in illa dispositione. Et sic non ordinabuntur ab eo partes unius rei vise nec distinguentur ab eo.

6.12 ◉ Accordingly, we will say that when the eye faces any visible object, the form of both the color and the light in that object will come from any point on its surface to the entire surface of the eye. Moreover, from every point on every visible object facing the eye under these circumstances the forms of the color and light in it will come to the entire surface of the eye. Hence, if the eye were to sense throughout its entire surface the forms of the color and light that come from any given point on the visible object’s surface, it would sense throughout its entire surface the form of every point on the surface of the visible object as well as the form of every point on the surfaces of all the visible objects facing it in that situation. So the parts of any one visible object would not be perceived according to their proper arrangement, nor would they be properly discerned by it.⁑

6.13 ◉ Et si senserit formam venientem ex uno puncto superficiei rei vise ad totam superficiem visus ex uno puncto tantum ex superficie ipsius visus, et non senserit formam illius puncti ex tota eius superficie, ordinabuntur ab eo partes rei vise, et distinguentur omnia visibilia opposita. Quoniam quando comprehenderit colorem puncti unius ex uno puncto tantum superficiei eius, comprehendet colorem unius partis rei vise ex una parte superficiei sue, et comprehendet colorem alterius partis ex alia parte superficiei sue. Et comprehendet unamquamque partem visibilium ex loco sue superficiei diverso ei per quem comprehendet aliam rem visibilem, quare visibilia erunt ab eo ordinata et distincta; et similiter partes cuiuslibet illorum.

6.13 ◉ But if the eye sensed at only one point on its surface the form reaching its entire surface from one point on the surface of the visible object, and if it did not sense the form of that point throughout its entire surface, the parts of the visible object would be perceived by it according to their proper arrangement, and all the facing visible objects would be properly discerned. The reason is that when it perceives the color of a single point at only one point on its surface, it will perceive the color of one part of the visible object at one part of its surface, and it will perceive the color of another part [of the object] at another part of its surface. And it will perceive each part of visible objects at a spot on its surface different from the spot where it will perceive another visible object; so [different] visible objects will be perceived by it in proper arrangement and distinctly, as will the parts of each of them.⁑

6.14 ◉ Modo ergo consideremus utrum hoc sit possibile et conveniens ad esse. Et dicamus prius quod visio non est nisi per glacialem, sive sit visio per formas venientes ex re visa ad visum sive secundum alium modum. Visio autem non est per unam aliarum tunicarum antecedentium sibi, quoniam ille tunice antecedentes non sunt nisi instrumenta glacialis. Quoniam si contingit humori glaciali occasio cum salute aliarum tunicarum, destruetur visio; et si acciderit residuis tunicis occasio remanente sua diafonitate cum salute glacialis, non cassabitur visus. Et iterum si in foramine uvee fuerit opilatio et destruatur diafonitas humoris eius, destruetur visus cum salute cornee; et si auferatur opilatio, revertetur visus. Et similiter si pervenerit intra humorem albugineum pars grossa non diafona, et fuerit in facie humoris glacialis et medians inter ipsum et foramen uvee, destruetur visio; et cum auferetur illud grossum vel declinabitur a verticatione recte que est inter glacialem et foramen uvee ad aliquam partem, revertetur visus. Et omnibus istis attestatur medicina.

6.14 ◉ So let us now consider whether this is possible and corresponds to reality. And we should say at the outset that vision takes place only through the glacialis, whether vision occurs by means of forms coming from the visible object to the eye or by some other means. Moreover, vision does not occur through one of the other tunics in front of it, for those tunics in front are only there to serve the glacialis. For if an injury happens to the glacial humor while the other tunics remain sound, vision will be extinguished; but if the remaining tunics suffer injury while retaining their transparency, and if the glacialis remains healthy, sight will not be disrupted. Likewise, if there is an obstruction in the opening of the uvea so that the capacity of its humor to transmit light is destroyed, sight will be extinguished, even when the cornea is healthy; but if the obstruction is removed, sight will be restored. So too, if a crass, nontransparent spot develops within the albugineous humor, and if it lies directly in front of the glacial humor between it and the opening of the uvea, vision will be extinguished; but when that dense spot is removed or turned aside from the straight line between the glacialis and the opening in the uvea, sight will be restored. And medical science attests to all these points.⁑

6.15 ◉ Destructio ergo sensus apud corruptionem glacialis cum salute tunicarum antecedentium illi est significatio quod sensus non est nisi per istum humorem, non per tunicas residuas antecedentes illi. Et destructio sensus apud destructionem diafonitatis que est inter glacialem et superficiem visus per corpus densum non translucens significat quod diafonitas istarum tunicarum non est nisi ut continuetur diafonitas tunicarum visus cum diafonitate aeris et efficiantur corpora que sunt inter glacialem et rem visam diafona continuate diafonitatis. Et destructio sensus apud abscisionem linearum rectarum que sunt inter glacialem et superficiem visus significat quod sensus glacialis non erit nisi ex lineis rectis que sunt inter ipsam et superficiem visus.

6.15 ◉ Therefore, the destruction of [visual] sensation that ensues from degeneration of the glacialis while the tunics in front of it remain healthy is an indication that [visual] sensation occurs by means of this humor alone, not by means of the rest of the tunics in front of it. Furthermore, the destruction of [visual] sensation that ensues from the disruption of the transparency between the glacialis and the eye’s surface by a crass, nontrans-parent body indicates that the transparency of these tunics exists only to link the transparency of the eye’s tunics with the transparency of the air so as to form a continuum of transparent media between the glacialis and the visible object. Also, the destruction of [visual] sensation when the straight lines between the glacialis and the eye’s surface are interrupted indicates that the glacialis will sense only along the straight lines between it and the surface of the eye.

6.16 ◉ Dicemus ergo si sensus visus ex colore rei vise et lucis que sunt in eo est ex forma veniente ex rebus visis ad superficiem visus, et sensus non est nisi per glacialem, ergo non per superficiem visus sentiet visus istam formam nisi postquam transierit superficiem visus et pervenerit ad glacialem. Et forma que venit ex re visa ad superficiem visus pertransit in diafonitate tunicarum visus, quoniam ex proprietate diafonitatis est ut transeant in ea forme lucis et colorum et extendantur recte. Et iam declaravimus hoc in aere; et cum fuerint experimentata omnia corpora diafona, invenietur quod lux non extendetur in eis nisi secundum lineas rectas. Et nos declarabimus post apud nostrum sermonem in obliquatione quomodo illud experimentabitur. Si ergo sensus visus lucis et coloris que sunt in re visa est ex forma veniente ad visum ex re visa, apud perventionem ipsius forme ad glacialem erit sensus. Et iam declaratum est quod non est possibile ut visus comprehendat rem visam secundum suum esse nisi quando comprehenderit formam unius puncti rei vise ex uno puncto tantum sue superficiei. Non est ergo possibile ut glacialis comprehendat rem visam secundum suum esse nisi quando comprehenderit colorem unius puncti rei vise ex forma perveniente ad ipsum ex uno puncto tantum superficiei visus. Forma autem venit ex quolibet puncto superficiei rei vise, et pertransit totam visus superficiem usque ad interius. Si vero ex eo quod venit ex uno puncto rei vise ad totam superficiem visus et pertransit tunicas visus et pervenit ad glacialem non comprehendit glacialis nisi quod venit ad ipsam ex uno puncto tantum superficiei visus, et sentit colorem illius puncti tantum ex superficie visus et pervenit ad unum punctum tantum superficiei eius, et non comprehendit illud punctum rei vise ex residua forma perveniente ad superficiem eius ex residua superficie visus, complebitur visio, et ordinabuntur partes rei vise, et distinguentur res vise apud visum.

6.16 ◉ We shall therefore say that, if the visual sensation of the color and light that are in a visible object arises from the form coming to the surface of the eye from visible objects, and if this sensation occurs by means of the glacialis alone, then sight will not sense that form at the surface of the eye itself but only after it passes through the eye’s surface and reaches the glacialis. And the form that reaches from the visible object to the eye’s surface passes through the transparency of the eye’s tunics, for it is among the properties of transparency that the forms of light and colors pass through it and continue rectilinearly. We have already made this point in regard to air; and if all transparent objects were to be tested, it would be found that light will extend through them only in straight lines.⁑ And in our discussion of the refraction [of light] we shall show how this point is to be experimentally confirmed.⁑ Therefore, if visual sensation of the light and color in a visible object is due to a form coming from that visible object to the eye, [that] sensation will arise [only] when that form itself reaches the glacialis. And it has already been shown that it is not possible for sight to perceive a visible object as it really exists unless it perceives the form of one point on the object at one point only on its own surface.⁑ So it is not possible for the glacialis to perceive a visible object as it really exists unless, from the form reaching it from the object, it perceives the color of one point on that visible object at one particular point on the surface of the eye. Now a form comes from any given point on the surface of the visible object, and it passes through the entire surface of the eye into its interior. If, however, the glacialis perceives only the form that reaches it at a single point on the surface of the eye from a single point on the visible object, this form having reached the entire surface of the eye and having passed through the tunics to the glacialis, and if it senses the color of that point alone that passes from the surface of the eye to [that] single point on its surface, and if it does not perceive that [same] point on the visible object from the rest of the form reaching its surface from the rest of the eye’s surface, then vision will be achieved, the parts of the visible object will be perceived according to their proper arrangement, and the visible objects will be properly discerned by sight.⁑

6.17 ◉ Et non complebitur visio nisi sit secundum illum modum. Et hoc non potest esse ita nisi quando fuerit unum punctorum que sunt in superficie visus per quam transit forma unius puncti superficiei rei vise distinctum a punctis residuis que sunt in superficie visus, et fuerit linea super quam venit forma ad illud punctum superficiei visus distincta a residuis lineis super quas venit forma. Et propter hoc potest glacialis comprehendere formam venientem super illam lineam et ex puncto superficiei visus qui est super illam lineam, et non potest comprehendere ipsam per aliam.

6.17 ◉ Moreover, vision will be achieved in this way alone. And such cannot be the case unless [each] one of the points on the surface of the eye through which the form of any one point on the surface of the visible object passes is distinct from the remaining points on the surface of the eye, and unless the line along which the form is radiated to that point on the surface of the eye is distinct from the remaining lines along which the form is radiated. Accordingly, the glacialis can perceive the form arriving along that line through the point on the surface of the eye that lies upon that line but cannot perceive it along any other.

6.18 ◉ Et cum inducantur luces et experimentetur qualitas transitus earum et extensionis earum in corporibus diafonis, invenitur quod lux extenditur per corpus diafonum secundum lineas rectas, dum corpus diafonum fuerit consimilis diafonitatis. Et cum occurrerit corpus aliud diverse diafonitatis a diafonitate precedentis corporis in quo extendebatur, non pertransibit secundum rectitudinem linearum super quas extendebatur ante nisi quando ille linee fuerint perpendiculares super superficiem secundi corporis diafoni. Et si ille linee fuerint obliquate super superficiem secundi corporis et non perpendiculares, obliquabitur lux apud superficiem secundi corporis, et non extendetur recte. Et cum obliquatur, extendetur in secundo corpore secundum illas lineas rectas super quas obliquabatur; et erunt linee super quas obliquabatur lux in secundo corpore etiam declinantes super superficiem secundi corporis et non perpendiculares. Et si fuerint quedam linee super quas venit lux in primo corpore perpendiculares super superficiem secundi corporis et quedam declinantes, extendetur lux que erit super lineas perpendiculares in corpore secundo secundum rectitudinem. Et que erit super lineas declinantes obliquabitur apud superficiem secundi corporis secundum lineas declinantes, et extendetur in eo secundum rectitudinem illarum linearum declinantium super quas obliquabatur. Et hoc nos declarabimus in sermone de obliquatione, et ostendemus viam per quam poterit quis experimentari istam dispositionem, et apparebit sensui, et cadet super ipsam certitudo.

6.18 ◉ And when lights are examined⁑ and the way they pass into and continue through transparent bodies is experimentally determined, it is found that light continues through a transparent body along straight lines, as long as the body is of consistent transparency. But when it strikes a body whose transparency is different from the transparency of the body through which it previously extended, it will not continue upon the straight lines along which it had extended before unless those lines are perpendicular to the surface of the second transparent body. If, however, those lines are oblique rather than perpendicular to the surface of the second body, the light will be bent at the surface of the second body rather than continue straight.⁑ And when it is bent, it will extend through the second body along those straight lines to which it has been inclined; and the lines along which the light has been bent in the second body will also be oblique rather than perpendicular to the surface of the second body. And if some of the lines along which the light reaches the first body are perpendicular to the surface of the second body and some inclined, the light that is orthogonally incident will extend straight through the second body. The light arriving along oblique lines, for its part, will be diverted along oblique lines at the surface of the second body, and it will extend rectilinearly through that body along those oblique lines into which it has been diverted. And we shall explain this in our discussion of bending, and we shall show how one can confirm this phenomenon experimentally, and it will [thus] be empirically ascertained.⁑

6.19 ◉ Et cum ita est, forma ergo lucis et coloris que veniunt ex quolibet puncto rei vise ad superficiem visus, quando pervenerit ad superficiem visus, nichil pertransibit ex eis per diafonitatem tunicarum visus secundum rectitudinem nisi illud quod erit super lineam rectam elevatam super superficiem visus secundum angulos rectos. Et illud quod fuerit super aliam lineam reflectetur et non pertransibit recte, quoniam diafonitas tunicarum visus non est sicut diafonitas aeris contingentis superficiem visus; et illud quod reflectitur ex istis formis reflectetur etiam super lineas declinantes, non super lineas perpendiculares extensas ex locis reflexionis. Et nulla linea recta exit ad aliquod punctum superficiei visus ab uno puncto superficiei rei vise ita quod sit perpendicularis ad superficiem visus nisi una linea tantum, et exeunt ad eam linee infinite declinantes super superficiem visus. Et forma veniens secundum rectitudinem perpendicularis pertransit tunicas visus secundum rectitudinem perpendicularis, et omnes forme venientes super lineas declinantes ad illud punctum reflectuntur ad illud punctum, et transeunt in tunicis visus secundum lineas declinantes etiam. Et nichil transit ex eis secundum latitudinem linearum super quas venerunt nec secundum rectitudinem perpendicularis erecti super illud punctum.

6.19 ◉ And since this is the case, when the form of the light and color that reach the surface of the eye from any given point on the visible object arrives at the surface of the eye, only the light and color that are incident at right angles upon the surface of the eye will pass straight through the transparency of the tunics of the eye. The form incident along any other line will be refracted⁑ and will not pass straight through, because the transparency of the tunics of the eyes is not the same as the transparency of the air contiguous with the surface of the eye; and those forms that are refracted will also be refracted along oblique lines rather than continuing along lines perpendicular to the [refracting surfaces] at the points of refraction. And there is only one straight line that extends from any single point on the surface of the visible object to a given point on the surface of the eye so as to be orthogonal to the surface of the eye, whereas there is an infinite number of lines extending to the surface of the eye that are inclined to it. And the form that arrives straight along the perpendicular passes straight through the tunics of the eye along the perpendicular, whereas all the forms incident to that [same] point along oblique lines are refracted at that point, and they pass through the tunics of the eye along oblique lines as well. None of them passes through along the same lines that they followed in arriving, nor [do they pass] straight through along the perpendicular erected at that point [of refraction].⁑

6.20 ◉ Et ad quodlibet punctum superficiei visus veniunt in eodem tempore forme omnium punctorum que sunt in superficiebus omnium visibilium illuminatorum oppositorum illi in illo tempore, quoniam inter ipsum et quodlibet punctum oppositum illi est linea recta. Et a quolibet punctorum que sunt in superficiebus visibilium illuminatorum extenduntur forme eius super quamlibet lineam rectam que potest extendi ex illo puncto, et forma unius puncti tantum de numero omnium punctorum oppositorum visui que venit ad illud punctum superficiei visus in illo tempore venit super perpendicularem elevatam super illud punctum superficiei visus. Et forme omnium punctorum residuorum veniunt ad illud punctum superficiei visus super lineas declinantes. Et in quolibet puncto superficiei visus transeunt in eodem tempore forme omnium punctorum que sunt in superficiebus omnium visibilium oppositorum in illo tempore. Et forma unius puncti tantum transit recte per diafonitatem tunicarum visus, et est punctus qui est apud extremitatem perpendicularis exeuntis ab illo puncto superficiei visus. Et forme omnium punctorum residuorum reflectuntur apud illud punctum superficiei visus, et transeunt per diafonitatem tunicarum visus secundum lineas declinantes super superficiem visus.

6.20 ◉ Moreover, at any given time, the forms of all the points on the surfaces of all illuminated visible objects facing the surface of the eye arrive simultaneously at any point on it, for there is a straight line between that point and any point facing it. Also, the forms from any one of the points on the surfaces of illuminated visible objects radiate along every straight line that can be extended from that point, but of all the points facing the eye whose forms are incident upon a given point on the surface of the eye [at any given time], there is only one at that time that arrives along the perpendicular erected to that point on the surface of the eye. The forms of all the remaining points reach that point on the surface of the eye along oblique lines. Furthermore, through any point on the surface of the eye the forms of all the points on the surfaces of all the visible objects facing the eye pass simultaneously. But the form of only one point passes straight through the transparency of the tunics of the eye, and that point is the one that lies at the endpoint of the perpendicular extending from the given point on the surface of the eye. The forms of all the remaining points are refracted at that point on the surface of the eye, and they pass through the transparency of the tunics of the eye along lines that are oblique with respect to the eye’s surface.

6.21 ◉ Et etiam ex quolibet puncto superficiei glacialis exit una linea tantum perpendicularis super superficiem visus. Et exeunt ex eo linee infinite ad superficiem visus, et erunt declinantes super ipsam. Punctum ergo superficiei glacialis ex quo exit perpendicularis super superficiem visus et pertransit foramen uvee, exeunt ab eo linee infinite que transeunt in foramen uvee, et perveniunt ad superficiem visus, preter illum perpendicularem.

6.21 ◉ Also, from any given point on the surface of the glacialis there extends only one line that is perpendicular to the surface of the eye. But there are an infinite number of lines extending from that point that will be oblique to the surface of the eye. Thus, besides the perpendicular itself, an infinite number of lines extends from the point on the surface of the glacialis where the perpendicular to the surface of the eye originates, and this perpendicular passes through the opening in the uvea; the rest of the lines also pass through the opening in the uvea and reach the surface of the eye.

6.22 ◉ Et extremitates omnium linearum exeuntium a puncto aliquo superficiei glacialis et transeuntium per foramen uvee et pervenientium ad superficiem visus et declinantium super illam, quando fuerint ymaginate reflecti secundum modum quem affirmat diversitas diafonitatis que est inter diafonitatem corporis cornee et diafonitatem aeris, perveniunt ad diversa loca et ad puncta diversa de numero punctorum que sunt in superficiebus visibilium oppositorum visui in uno tempore. Et nulla istarum linearum occurrit puncto quod est apud extremitatem perpendicularis. Et forme punctorum que sunt apud extremitates omnium istarum linearum superficierum visibilium extenduntur secundum rectitudinem istarum linearum, et perveniunt ad superficiem visus, et reflectuntur ad idem punctum superficiei glacialis, preter formam puncti quod est apud extremitatem perpendicularis, quoniam ipsa extenditur secundum rectitudinem perpendicularis et pertransit ad illud punctum glacialis. Si ergo glacialis sentit ex uno puncto in eo omnes formas venientes ad ipsum ex omnibus verticationibus, sentiet ex omni puncto formas admixtas ex multis formis diversis et coloribus multis visibilium oppositorum visui in illo tempore. Et sic nichil distinguetur ab eo ex punctis que sunt in superficiebus visibilium nec ordinabuntur forme punctorum venientes ad illud punctum. Et si glacialis senserit ex uno puncto eius illud quod venit ad ipsum ex una verticatione tantum, distinguentur ab eo puncta que sunt in superficiebus visibilium.

6.22 ◉ Furthermore, if we suppose these lines to be refracted according to the way determined by the difference in transparency between the transparency of the corneal body and the transparency of the air, then the endpoints of all the lines that extend from any given point on the surface of the glacialis and pass through the opening of the uvea to reach the surface of the eye along oblique paths reach different locations and different points among the set of [all] points on the surfaces of visible objects that face the eye at any given time. And none of these lines intersects the point at the end of the perpendicular. So the forms of the points that lie on the surfaces of the visible objects at the extremities of all these lines are propagated rectilinearly along these lines, and they reach the surface of the eye where they are refracted to the same point on the surface of the glacialis, except for the form of the point lying at the extremity of the perpendicular, for it extends straight along the perpendicular and passes [straight through] to that point on the glacialis. Thus, if at any one of its points the glacialis senses all the forms reaching that point along all the lines of radiation,⁑ at every point it will sense forms that are mixed together from many different forms and many [different] colors [extending] from the visible objects that face the eye at that time. Hence, on the basis of this [mixed form] it will discern none of the [individual] points on the surfaces of those visible objects, nor will the forms of those points that reach that point be perceived according to their proper arrangement. Yet if the glacialis were to sense at one of its points only the form that reaches it along one particular radial line, the [individual] points on the surfaces of the visible objects would be properly discerned by it.

6.23 ◉ Et nullum punctorum quorum forme perveniunt ad glacialem super lineas reflexas est dignius alio ex formis reflexis, nec ulla verticatio reflexa est dignior alia; et forme reflexe ad unum punctum glacialis in uno tempore sunt multe, non determinate. Et punctum cuius forma venit secundum rectitudinem perpendicularis ad unum punctum glacialis fuit unum punctum tantum; et nulla alia forma venit cum ea secundum rectitudinem perpendicularis, quoniam omnes forme reflexe non reflectuntur nisi secundum lineas declinantes. Et etiam cum centrum superficiei visus sit idem cum centro superficiei glacialis, linea que est perpendicularis super superficiem visus est perpendicularis super superficiem glacialis. Forma ergo que venit super perpendicularem distinguitur ab aliis formis duabus dispositionibus quarum altera est quod ipsa extenditur a superficie rei vise ad punctum glacialis super lineam rectam, et residue veniunt super lineas reflexas. Altera autem est quod ipsa perpendicularis erecta super superficiem visus est perpendicularis super superficiem glacialis etiam, et linee residue super quas veniunt forme residue reflexe sunt declinantes super superficiem glacialis, quoniam sunt declinantes super superficiem visus.

6.23 ◉ But none of the points whose forms reach the glacialis along refracted lines is more exceptional than any of the other points whose forms are refracted, nor is any refracted path more exceptional than any other; and the forms that are refracted at any given point on the glacialis at any given time are innumerable. On the other hand, the point whose form reaches any one point on the glacialis along the perpendicular is unique; no other form accompanies it straight along the perpendicular, for all the forms that are refracted are refracted along oblique lines alone. Moreover, since the center of the eye’s surface coincides with the center of the glacialis’ surface, any line that is perpendicular to the surface of the eye is perpendicular to the surface of the glacialis. Hence, the form that arrives along the perpendicular is distinguished from all the other forms in two respects, the first of which is that it extends from the surface of the visible object to the point on the glacialis along a straight line, whereas the remaining forms reach [that point] along refracted lines. The second is that the perpendicular dropped to the surface of the eye is the very same perpendicular that is dropped to the surface of the glacialis, whereas the rest of the lines along which the remaining forms that are refracted reach [the eye] are oblique to the surface of the glacialis, because they are oblique to the surface of the eye.

6.24 ◉ Et operatio lucis venientis super perpendiculares est fortior operatione lucis venientis super lineas inclinatas. Dignius ergo est ut glacialis non sentiat ex quolibet puncto nisi formam venientem ad ipsum secundum rectitudinem perpendicularis tantum, et non sentiat ex illo puncto illud quod venit ad illud punctum secundum verticationes reflexas.

6.24 ◉ Furthermore, the effect of light arriving along perpendiculars is stronger than the effect of light arriving along oblique lines. Therefore, it is quite fitting that at any given point the glacialis senses only the form reaching it straight along the perpendicular and does not sense any form that strikes it at that point along refracted lines.⁑

6.25 ◉ Et etiam cum centrum superficiei visus et centrum superficiei glacialis sit idem punctum, omnes perpendiculares elevate super superficiem glacialis et superficiem visus concurrunt super centrum commune, et erunt dyametri in superficiebus tunicarum visus. Et erit quelibet perpendicularis occurrens superficiei cornee in uno puncto et occurrens superficiei glacialis in uno puncto, et non exit ad illud punctum cornee nisi una perpendicularis tantum, nec exit ad illud punctum glacialis nisi illa perpendicularis tantum. Forma ergo que exit ex quolibet puncto superficiei rei vise super perpendicularem que extenditur ab eo ad superficiem visus occurrit superficiei visus super unum punctum super quod non occurrit ei aliqua alia formarum venientium non super perpendiculares. Et etiam iam declaratum est quod ex quolibet puncto cuiuslibet corporis colorati et illuminati cum quolibet lumine exeunt lux et color super quamlibet lineam rectam que poterit extendi ab illo puncto.

6.25 ◉ In addition, since the center of the eye’s surface and the center of the surface of the glacialis coincide, all of the perpendiculars erected to the surface of the glacialis as well as to the surface of the eye intersect at that common center, and they will form diameters for the tunics of the eye. And every perpendicular will strike the surface of the cornea at one point and will strike the surface of the glacialis at one point, but at that point on the cornea only one perpendicular can be dropped, and at that point on the glacialis no perpendicular other than that one can be dropped. So the form that extends from any given point on the surface of the visible object along the perpendicular dropped from it to the surface of the eye strikes the surface of the eye at one point, but none of the other forms arriving [from that point on the visible object] along nonperpendicular lines strikes [the surface of the eye at] that particular point. Furthermore, it has already been shown that from any point on any colored body that is somehow illuminated light and color emanate along every straight line that can be extended from that point.⁑

6.26 ◉ Inter ergo quodlibet punctum oppositum alicui superficiei et quodlibet punctum illius superficiei est linea recta ymaginabilis, et inter illud punctum et totam illam superficiem est piramis ymaginabilis cuius conus est illud punctum et cuius basis est illa superficies. Et illa piramis continet omnes lineas rectas ymaginatas que sunt inter illud punctum et omnia puncta illius superficiei.

6.26 ◉ Therefore, one can imagine a straight line [extended] between any point facing a given surface and any point on that surface, and between that point and that whole surface a cone can be imagined with its vertex at that point and its base formed by that surface. And that cone contains all the straight lines that are imagined to lie between that [vertex-]point and all the points on that surface.⁑

6.27 ◉ Cum ergo forma lucis et coloris exierint a quolibet puncto superficiei corporis colorati illuminati super quamlibet lineam rectam que poterit extendi ab illo puncto ad quodlibet punctum oppositum corpori illuminato et colorato, forma lucis et coloris que sunt in superficie illius corporis extenditur a quolibet puncto superficiei illius corporis ad illud punctum oppositum illi super lineam rectam extensam inter ipsum corpus et illud punctum. Forma ergo lucis et coloris cuiuslibet corporis colorati et illuminati cum quolibet lumine extenditur a sua superficie ad quodlibet punctum oppositum illi superficiei secundum verticationem piramidis que formatur inter illud punctum et illam superficiem. Et erit forma ordinata in illa piramide per illas lineas concurrentes ad illud punctum qui est conus piramidis sicut est ordinatio in partibus coloris qui est in superficie illius corporis.

6.27 ◉ Accordingly, since the form of light and color radiates from any point on the surface of a colored and illuminated body along every straight line that can be extended from that point to any point facing that illuminated and colored body, the form of the light and color on that body’s surface is radiated from any point on the surface of that body to that facing point along a straight line extending between that same body and that point. The form of the light and color of any colored body that is somehow illuminated thus extends from its surface to any point facing that surface along a line contained by the cone that is formed between that point and that surface. And the form will be arranged within that cone according to the lines that intersect at that point, which forms the cone’s vertex, and that arrangement will be the same as the arrangement of the spots of color on the surface of that body.

6.28 ◉ Cum ergo fuerit visus oppositus alicui rei visibili, formabitur inter punctum qui est centrum visus et superficiem illius rei vise piramis ymaginata cuius conus erit centrum visus et basis erit superficies illius rei vise. Et cum aer medians inter illam rem visam et visum fuerit continuus, et non fuerit medium inter rem visam et visum corpus densum, et fuerit illa res visa illuminata cum quolibet lumine, erit forma lucis et coloris que sunt in superficie illius rei vise extensa ad visum secundum verticationem illius piramidis. Et erit forma cuiuslibet puncti superficiei illius rei vise extensa secundum rectitudinem linee que est inter illud punctum et conum piramidis quod est centrum visus.

6.28 ◉ So when the eye faces any visible object, a cone can be conceived of as formed between the point that represents the center of the eye and the surface of that visible object, the vertex of that cone being the center of the eye and its base being the surface of that visible object. And if the intervening air between that visible object and the eye is continuous, if there is no opaque body interposed between that visible object and the eye, and if that visible object is somehow illuminated, the form of the light and color on the surface of that visible object will reach the eye along a line contained by that cone. And the form of every point on the surface of that visible object will radiate along the straight line connecting that point and the vertex of the cone, which lies at the center of the eye.

6.29 ◉ Et quia centrum visus idem est cum centro superficiei glacialis, erunt omnes iste linee perpendiculares super superficiem manifesti oculi, et super superficiem glacialis, et super omnes superficies visus equidistantes. Et erit piramis continua super omnes istas perpendiculares continens omnes istas perpendiculares et aerem in quo extenditur forma a tota superficie illius rei vise opposite visui secundum verticationes perpendicularium. Et erit superficies glacialis secans istam piramidem, et sic pervenit forma lucis et coloris que sunt in superficie illius rei vise in partem superficiei glacialis quam distinguit piramis. Et ad quodlibet punctum istius partis superficiei glacialis veniet forma puncti oppositi superficiei rei vise secundum rectitudinem perpendicularis exeuntis ab illo puncto superficiei rei vise super superficies tunicarum visus et super superficiem glacialis. Et pertransit diafonitatem tunicarum visus secundum rectitudinem illius perpendicularis, et non pertransit cum illa forma secundum rectitudinem illius perpendicularis alia forma. Et erit ista forma perveniens ad istam partem glacialis ordinata in ea secundum lineas super quas pervenit ad ipsam que sunt perpendiculares ad ipsam et concurrentes apud centrum visus sicut ordinatio partium superficiei rei vise. Et cum hoc veniunt in illa dispositione ad quod libet punctum huius partis superficiei glacialis multe forme a multis punctis superficierum visarum in eodem tempore. Perveniunt ergo in ista parte superficiei glacialis que distinguebatur a piramide multe forme ex multis coloribus diversis.

6.29 ◉ Furthermore, since the center of the eye[‘s surface] is the same as the center of the surface of the glacialis, all of these lines will be perpendicular to the outside surface of the eye as well as to the surface of the glacialis and all the surfaces of the eye that are parallel [to them]. And the cone that coincides with all these perpendicular lines will encompass all these perpendiculars and the air through which the form [in its entirety] extends along perpendicular lines from the whole surface of that visible object facing the eye. Also, the surface of the glacialis will intersect that cone, so the form of the light and color on the surface of that visible object reaches the section of the glacialis that is demarcated by the cone. At any point on this section of the surface of the glacialis the form of a corresponding point on the surface of the visible object will arrive along the perpendicular dropped from that point on the surface of the visible object to the surfaces of the tunics of the eye as well as to the surface of the glacialis.⁑ And this form passes straight through the transparency of the tunics of the eye along that perpendicular, but no other form passes straight through in tandem with that form along that perpendicular line. That form, moreover, will reach this spot on the glacialis according to the arrangement determined by the lines along which it arrives there, those lines being perpendicular to the glacialis and intersecting at the center of the eye in an arrangement corresponding to that of the parts of the surface of the visible object.⁑ Furthermore, under these circumstances several forms reach any point on this section of the glacialis from several points on the visible surfaces at the same time. Thus, several forms arising from several different colors reach this section of the glacialis that has been demarcated by the cone.

6.30 ◉ Si ergo glacialis senserit ex parte distincta per piramidem formam venientem ad ipsam ex verticatione illius piramidis tantum, nec non senserit ex illa parte sue superficiei aliam formam nisi formam venientem super illam verticationem, sentiet formam illius rei secundum suum esse, et sentiet ordinatam secundum suam ordinationem. Et poterit etiam sentire in illa dispositione formas aliarum rerum visarum, preter illam rem visam, ex piramidibus distinguentibus ex sua superficie alias partes ab illa parte, et poterit sentire formam cuiuslibet illarum rerum visarum secundum suum esse et sentire situs earum adinvicem secundum suum esse.

6.30 ◉ If, therefore, the glacialis senses the form reaching it at one, distinct point along only one of the lines within that cone, and if at that [same] spot on its surface it senses no other form than the form reaching it along that line, then it will sense the form of that object as it actually exists, and it will sense it according to its arrangement [on the visible surface]. Moreover, under those circumstances, it will be able to sense the forms of visible objects other than that visible object on the basis of the cones that demarcate other sections upon its surface, and it will be able to sense the form of each of those visible objects as they actually exist as well as to sense their relative locations as they actually exist.

6.31 ◉ Et si glacialis senserit formas venientes ad ipsam ex verticationibus reflexis, sentiet ex eadem parte que distinguebatur ex sua superficie per illam piramidem formas admixtas ex formis partium illius rei vise et ex formis multarum rerum visarum diversarum, et erunt admixte ex multis coloribus diversis. Et sentiet ex qualibet parte sue superficiei, preter illam partem, formam admixtam ex formis multarum rerum visarum diversarum, et sic non sentiet formam venientem secundum piramidis verticationem secundum suum esse, nec aliquam formarum venientium super perpendiculares secundum suum esse, nec aliquam formarum venientium ex verticationibus reflexis. Non sentiet ergo formam unius rei vise secundum suum esse, nec distinguentur ab ea res vise opposite illi in eodem tempore.

6.31 ◉ But if the glacialis senses forms arriving at it along refracted lines, the forms it will sense at the same section on its surface that was cut by the cone will be mixed from the forms of parts of the given visible object as well as from the forms of many different visible objects, and those forms will represent mixtures of many different colors. Moreover, at some spot on its surface other than that one it will sense a form that is mixed from the forms of many different visible objects, and so it will not sense the form reaching it along the line within the cone as it actually exists, nor will it sense any of the forms reaching it along the perpendiculars as they actually exist, nor will it sense any of the forms reaching it along refracted lines [as they actually exist]. Hence, it will not sense the form of any individual visible object as it actually exists, nor will visible objects facing it at any given time be [individually] discerned by it.

6.32 ◉ Sed visus comprehendet res visas distinctas, et comprehendet partes unius rei vise ordinatas secundum suum esse in superficie rei vise, et comprehendet res visas multas simul in eodem tempore. Et cum visio est ex formis venientibus ex rebus visis ad visum, nichil sentiet glacialis ex formis rerum visarum ex verticationibus reflexis.

6.32 ◉ But sight will [in fact] perceive separate visible objects, and it will perceive the parts of an individual visible object according to their actual arrangement on the surface of the visible object, and it will perceive several visible objects together at the same time. And since vision is due to forms reaching the eye from visible objects, the glacialis will sense none of the forms of visible objects that reach it along refracted lines.

6.33 ◉ Et etiam nulla formarum pervenientium ad superficiem glacialis ex formis rerum visarum ordinabitur in superficie glacialis secundum suum esse, nec ulla formarum partium unius rei vise pervenientium ad superficiem glacialis ordinabitur in superficie glacialis secundum suum esse in superficie rei vise nisi forme pervenientes ad eam secundum rectitudinem perpendicularium elevatarum super superficiem visus tantum. Situs autem formarum reflexarum apud superficiem visus etiam perveniunt in superficiem glacialis conversi. Et pervenit forma unius puncti cum hoc in portione superficiei glacialis, non in uno puncto, et illud est quia forma puncti dextri apud visum, quando extendetur ad punctum superficiei visus, et fuerit linea super quam extendetur forma declinans super superficiem visus, reflectetur ad partem sinistram a perpendiculari que extenditur a centro visus ad illud punctum sue superficiei. Et pervenit forma que reflectitur ab extremitate perpendicularis secundum hunc modum ad punctum sinistrum a puncto superficiei glacialis super quod abscindit illam illa perpendicularis. Et similiter forma puncti sinistri a visu que extenditur ad illud idem punctum superficiei visus et declinat super ipsam reflectetur ad punctum dextrum a perpendiculari et a puncto superficiei glacialis quod est super illam perpendicularem. Quoniam forme reflexe non appropinquant post reflexionem perpendiculari exeunti a loco reflexionis, et non perveniunt per applicationem forme perpendiculari, nec pertranseunt ipsam nec procedunt, quoniam hec est proprietas formarum reflexarum.

6.33 ◉ Furthermore, none of the forms reaching the surface of the glacialis from visible objects will be arranged on the surface of the glacialis according to reality, and none of the forms reaching the surface of the glacialis from the parts of the individual object will be arranged on the surface of the glacialis according to reality except for the forms reaching it directly along the perpendiculars dropped to the surface of the eye. The forms, moreover, that are refracted at the surface of the eye reach the surface of the glacialis in reverse order. And in addition to that, the form of one point is spread out upon an area of the surface of the glacialis rather than arriving at a point, and this follows from the fact that when the form of a right-hand point with respect to the eye reaches a point on the surface of the eye, assuming that the line along which that form extends is oblique to the eye’s surface, it will refract to the left of the normal dropped from the center of the eye to that point on its surface. And the form that is refracted in this way at the extremity of the normal [at the point of refraction] reaches a point to the left of the point on the surface of the glacialis where that perpendicular intersects it. So too, the form of a left-hand point with respect to the eye that extends to that same point [of refraction] on the surface of the eye and that is oblique to this surface will be refracted to a point on the right of the normal as well as on the right of the point on the surface of the glacialis that lies on that normal. For after refraction, refracted forms do not incline along the normal dropped to the point of refraction, nor do those forms coincide with the normal, nor do they pass through it or continue by it, for such is the property of refracted forms.⁑

6.34 ◉ Et similiter forme duorum punctorum que sunt in eadem parte a visu que exeunt ad unum punctum superficiei visus et declinant super ipsam in eadem parte perveniunt in superficie glacialis converse, quoniam due linee super quas extenduntur due forme punctorum secant se ad punctum superficiei visus super quod concurrunt due forme, et occurrunt perpendiculari exeunti ad illud punctum superficiei visus super illud punctum. Cum ergo iste due linee fuerint declinantes a superficie visus in eadem parte a perpendiculari exeunti a centro visus ad illud punctum, reflectuntur forme duorum punctorum ad partem oppositam illi parti. Et etiam quia due linee super quas extenduntur due forme ad unum punctum superficiei visus secant se super illud punctum, oportet quod, quando extenduntur secundum suam rectitudinem post sectionem, ut appareat situs eorum conversus in respectu eius qui est in re visa et etiam respectu perpendicularis. Et efficitur linea que erat dextra ante suam perventionem ad superficiem visus ex illis duabus lineis sinistra post suum pertransitum in superficie visus et sinistra dextra.

6.35 ◉ Et similiter erit situs duarum linearum super quas reflectentur due forme ex uno puncto superficiei visus, quoniam due forme que reflectuntur ex uno puncto appropinquant ambo perpendiculari, et extenditur forma que erat super lineam remotiorem a perpendiculari post sectionem super lineam remotiorem etiam a perpendiculari, sed minoris remotionis quam linea super quam erat. Et extenditur forma que erat super lineam propinquiorem perpendiculari etiam post sectionem super lineam propinquiorem perpendiculari etiam, sed maioris propinquitatis quam linea super quam erat, et similiter omnes forme que reflectuntur ab uno puncto.

6.35 ◉ The same will hold for the [relative] position of the two lines along which the two forms are refracted at one point on the surface of the eye, for the two forms that are refracted at one point both approach the normal, and, after intersecting [the normal], the form that arrived along the line farther from the normal continues along a line that is also farther from the normal, but less so than before. Meantime, after intersecting [the normal], the form that arrived along the line nearer to the normal still continues along a line that is nearer the normal, but more so than before, and the same holds for all forms that are refracted at a single point.⁑

6.36 ◉ Et cum hoc fuerit experimentatum experimentatione subtili, invenietur secundum quod diximus. Et nos ostendemus viam per quam experimentabitur hoc experimentatione vera apud nostrum sermonem in reflexione, et tunc discooperientur omnia dependentia a reflexione. Et nos non utemur illic in declaratione rerum quibus usi fuimus in isto tractatu ex illis que declaravimus in isto tractatu per istas res.

6.36 ◉ And if this phenomenon is experimentally scrutinized with great care, the result will be found to agree with what we have claimed. And we shall show how to carry out this experimental confirmation properly in our section on refraction,⁑ and at that time everything to do with refraction will be revealed. But in that section we shall not avail ourselves of the discussion of matters that we have demonstrated in this book concerning such phenomena.

6.37 ◉ Duo ergo puncta declinantia ad unam partem a re visa, quando forme eorum extenduntur ad unum punctum superficiei visus, secabunt se super duas lineas quarum situs erit apud visum in respectu rei vise econverso a situ duarum linearum primarum super quas extendebantur due forme ad superficiem visus. Erit ergo situs duorum punctorum superficiei glacialis ad que perveniunt due forme econtrario situi duorum punctorum ex quibus veniunt due forme. Omnes ergo forme que reflectuntur ab uno puncto superficiei visus perveniunt in superficie glacialis converse.

6.37 ◉ Therefore, when the forms of two points on one side of a visible object arrive obliquely at a single point on the surface of the eye, they will intersect [and continue] along two lines whose [relative] position with respect to the visible object from the perspective of the viewer will be opposite to the [relative] position of the two lines along which the two forms originally reached the surface of the eye. Accordingly, the position of the two points on the surface of the glacialis to which the two forms reach will be opposite the position of the two points [on the surface of the visible object] from which the two forms originate. All forms that are refracted at one point on the surface of the eye thus arrive in reverse order on the surface of the glacialis.⁑

6.38 ◉ Et etiam forma cuiuslibet puncti oppositi visui venit ad totam superficiem visus; igitur reflectetur a tota superficie visus. Et forma que reflectitur a tota superficie visus reflectitur ad partem alicuius quantitatis superficiei glacialis, non ad unum punctum, quoniam forme reflexionis, si concurrerint post reflexionem super unum punctum, secarent perpendiculares apud quarum extremitates reflectebantur, aut pertransirent ipsas, aut exiret forma a superficie in qua reflectebatur. Sed nulla forma reflexa occurrit perpendiculari apud cuius extremitatem fuit reflexa post reflexionem, nec pertransit illam, nec exit a superficie in qua fuit reflexa. Et omnia ista manifestantur apud experimentationem. Forma ergo unius puncti rei vise que pervenit in superficie glacialis per reflexionem non erit in uno puncto sed in parte alicuius quantitatis superficiei glacialis. Et non erit situs formarum punctorum diversorum superficiei rei vise que perveniunt in superficie glacialis per reflexionem adinvicem sicut situs earum secundum suum esse in superficiebus rerum visarum, sed econtrario. Nulla ergo formarum reflexarum rerum visarum pervenientium ad superficiem glacialis est secundum suum esse in superficiebus rerum visarum. Et iam declaratum est quod forme venientes super perpendiculares ordinantur in superficie glacialis secundum suum esse, quoniam extenduntur recte a superficiebus rerum visarum ad superficiem glacialis. Nulla ergo formarum rerum visarum venientium ad superficiem glacialis ordinatur in superficie glacialis secundum suum esse in superficiebus rerum visarum nisi forme extense super verticationes perpendicularium tantum.

6.38 ◉ Furthermore, the form of any point facing the eye reaches the entire surface of the eye; hence it will be refracted at the entire surface of the eye. And the form that is refracted at the entire surface of the eye is refracted to an area on the surface of the glacialis that has some dimension, not to a point, for if refracted forms were to meet at one point after refraction, they would either intersect or pass through the normals at whose endpoint they have been refracted, or the form would pass out of the plane within which it has been refracted.⁑ But, after being refracted, no refracted form meets the normal at whose endpoint it was refracted, nor does it pass through it, nor does it pass out of the plane within which it was refracted. And all of these points become clear with experimentation. Therefore, [when] the form of a single point on a visible object reaches the surface of the glacialis through refraction, [it] will not reach it at a single point but, rather, at an area on the surface of the glacialis that has some dimension. Moreover, the relative positions of the forms of different points on the surface of the visible object that reach the surface of the glacialis through refraction will not be the same as their actual positions on the surfaces of [those] visible objects, but reversed. Thus, none of the refracted forms of visible objects reaching the surface of the glacialis represents the surfaces of the visible objects as they actually are. But it has already been shown that forms arriving along perpendiculars are arranged on the surface of the glacialis according to reality, because they extend orthogonally from the surfaces of visible objects to the surface of the glacialis. Except for the forms extending along perpendicular lines, then, none of the forms of visible objects that reach the surface of the glacialis is arranged on the surface of the glacialis according to its actual arrangement on the surfaces of visible objects.

6.39 ◉ Si ergo sensus rerum visarum sit ex formis venientibus ad ipsum ex superficiebus rerum visarum, nichil comprehendet visus ex formis rerum visarum pervenientium ad ipsum nisi ex verticationibus linearum quarum extremitates concurrunt apud centrum visus tantum, quoniam visus nichil comprehendit ex formis rerum visarum nisi ordinatum secundum suum esse in superficiebus rerum visarum.

6.39 ◉ Hence, if visible objects are sensed by means of forms reaching the eye from the surfaces of visible objects, sight will perceive none of the forms of visible objects that reach it along lines other than those whose endpoints meet at the center of the eye, for sight perceives none of the forms of visible objects unless they are arranged [in sight] according to their actual arrangement on the surfaces of visible objects.

6.40 ◉ Et etiam cum centrum superficiei visus non est centrum superficei glacialis, linee recte que exeunt a centro superficiei visus, et extenduntur in foramine uvee, et perveniunt ad res visas non erunt perpendiculares super superficiem glacialis sed declinantes super ipsam; nec situs earum super superficiem glacialis erunt situs consimiles nisi una linea tantum, et est que transit per duo centra. Formas ergo venientes a superficiebus rerum visarum ad superficiem glacialis non potest sentire glacialis nisi ex verticationibus istarum linearum tantum—scilicet que sunt perpendiculares super superficiem visus que est superficies cornee. Quoniam forme que sunt super istas perpendiculares tantum sunt ordinate in superficie glacialis secundum ordinationem earum in superficiebus rerum visarum.

6.40 ◉ Moreover, if the center of the eye[ball]’s surface is not [the same as] the center of the surface of the glacialis, then the straight lines originating at the center of the surface of the eye that extend through the opening in the uvea and reach visible objects will be oblique rather than perpendicular to the surface of the glacialis; and their [relative] positions on the surface of the glacialis will not be constant, except for one line alone, and that is the one that passes through both centers. Therefore, the glacialis can only sense the forms reaching its surface from the surfaces of visible objects along those lines alone—i.e., the lines that are perpendicular to the surface of the eye, which is the surface of the cornea. For only the forms that lie upon these perpendiculars are arranged on the surface of the glacialis according to their arrangement on the surfaces of visible objects.

6.41 ◉ Si ergo glacialis comprehendat res visas ex formis venientibus ad ipsam et non comprehendat formam nisi ex verticationibus istarum linearum, et iste linee non sunt perpendiculares super superficiem eius, comprehendet tunc formas ex verticationibus quarum situs a sua superficie sunt diversi situs et declinantes super suam superficiem. Et comprehendit formas ex verticationibus diversorum situum declinantibus, et comprehendet omnes formas reflexas ex verticationibus diversorum situum apud suam superficiem. Et si comprehendisset omnes formas reflexas ex verticationibus diversorum situum, nichil distinguetur ab ea ex rebus visis, propter hoc quod de claratum est superius. Et cum non est possibile ut comprehendat formas reflexas ex verticationibus diversorum situum, non est possibile ut comprehendat formas rerum visarum ex verticationibus linearum que sunt perpendiculares super superficiem visus nisi quando linee iste fuerint perpendiculares super superficiem eius et fuerint situs eorum in superficie eius consimiles. Et iste linee non erunt perpendiculares super superficiem suam nisi quando centrum sue superficiei et centrum superficiei visus fuerint idem punctum. Si ergo sensus visus rerum visarum est ex formis venientibus ad ipsum ex coloribus rerum visarum et lucibus earum, oportet ut centrum superficiei visus et centrum superficiei glacialis sit unum punctum commune, et nichil comprehendat visus ex formis rerum visarum nisi ex verticationibus linearum rectarum quarum extremitates concurrunt apud istud centrum tantum.

6.41 ◉ If, then, the glacialis perceives visible objects by means of forms reaching it and perceives only that form reaching it along these lines, and if these lines are not perpendicular to its surface, then it will perceive forms along lines that are oblique to its surface and whose [relative] locations are variable with respect to its surface. So it perceives forms along oblique lines that have different [relative] positions, and it will perceive all refracted forms along lines that have different [relative] positions with respect to its surface. But if it were to perceive all refracted forms along lines that have different [relative] positions, none of the visible objects would be [individually] discerned by it, according to what has been demonstrated above.⁑ And if it is not possible for the glacialis to perceive refracted forms of visible objects along lines that have different [relative] positions, it is not possible for it to perceive the forms of visible objects along lines that are perpendicular to the surface of the eye unless those lines are perpendicular to its surface and unless their [relative] position on its surface is constant. But these lines will only be perpendicular to the surface of the glacialis if the center of its surface is the same point as the center of the surface of the eye. Thus, if the visual sensation of visible objects is due to forms reaching the eye from the colors and light of [those] visible objects, then the center of the eye’s surface and the center of the surface of the glacialis must be a single, common point, and sight can perceive none of the forms of visible objects unless it does so exclusively along the straight lines whose endpoints meet at this centerpoint.

6.42 ◉ Et non est impossibile ut duo centra sint idem, quoniam iam declaratum est quod duo centra sunt ex posteriori centro uvee et sunt super unam lineam rectam transeuntem per omnia centra. Et cum non est impossibile ut duo centra sint idem et ut linee recte que exeunt a centris sint perpendiculares super duas superficies—scilicet super superficiem glacialis et superficiem visus—non est etiam impossibile ut sit comprehensio visus rerum visarum ex formis venientibus ad ipsum coloris et lucis que sunt in superficie rerum visarum, cum comprehensio istarum formarum sit ex verticationibus perpendicularium tantum. Et illud est ut natura visus sit recipiens ea que veniunt ad ipsum ex formis rerum visarum, et etiam ut sit natura visus cum hoc appropriata ut non recipiat ea que veniunt ad ipsum ex formis nisi ex propriis verticationibus, non ex omnibus verticationibus; et sunt verticationes linearum rectarum quarum extremitates concurrunt apud centrum visus tantum, et iste linee appropinquantur in centro quia sunt dyametri (eius visus scilicet) et perpendiculares super superficiem sentientis. Et sic erit sensus ex formis venientibus ex rebus visis, et erunt iste linee quasi instrumentum visus per quod distinguentur a visu res vise et per quod ordinabuntur partes cuiuslibet rerum visarum.

6.42 ◉ Now it is not impossible for the two centers to coincide, for it has already been shown that the two centers lie behind the center of the uvea upon a single straight line that passes through all the centers [of the tunics of the eye].⁑ And since it is not impossible for the two centers to be the same and for the straight lines passing through the centers to be perpendicular to the two surfaces—i.e., the surface of the glacialis and the surface of the eye—then it is also not impossible for the visual perception of visible objects to be due to the forms of light and color coming to it from the surfaces of [those] visible objects, if the perception of those forms takes place along perpendicular lines alone. And this is so because it is in the nature of sight to receive forms that reach it from visible objects, and also because, in addition to this qualification, it is in the nature of sight only to accept those forms that reach it along specific lines, not along all lines; and these specific lines are the straight lines alone whose endpoints meet at the center of the eye, and these lines converge at the center because they are diameters (of the eye, that is) and are perpendicular to the surface of the sensing organ. And so [visual] sensation will be due to the forms that come from visible objects, and the perpendicular lines [along which they are sensed] will be, as it were, the instrument of sight by means of which visible objects will be [individually] discerned by sight and the parts of every visible object will be [properly] arranged [for visual perception].

6.43 ◉ Et quod esse visus appropriatur aliquibus verticationibus tantum habet similia in rebus naturalibus. Quoniam lux oritur ex corporibus luminosis, et extenditur super verticationes rectas tantum, et non extenditur super lineas arcuales aut curvas; et corpora ponderosa moventur ad inferius motu naturali super lineas rectas, non super lineas curvas aut arcuales aut tortuosas. Et non movebuntur super omnes lineas rectas que sunt inter eas et superficiem terre, sed super lineas rectas proprias que sunt perpendiculares super superficiem terre et dyametrum terre. Et corpora celestia moventur super lineas spericas et non super lineas rectas nec super lineas diversi ordinis. Et cum fuerimus intuentes motus naturales, inveniemus quod quilibet eorum est appropriatus aliquibus verticationibus. Non est ergo impossibile ut sit visus appropriatus in receptione operationum lucis et coloris aliquibus verticationibus rectis que concurrunt apud eius centrum tantum, et sunt super perpendiculares super superficiem eius. Comprehensio autem visus de rebus visis ex verticationibus linearum rectarum quarum extremitates concurrunt apud centrum visus est concessum a mathematicis, et nulla diversitas est inter eos in hoc. Et iste linee vocantur ab eis linee radiales.

6.43 ◉ Moreover, the fact that sight functions according to particular lines has counterparts in [other] natural phenomena. For light originates at luminous bodies and extends along straight lines only rather than following curved or crooked lines; and heavy bodies fall naturally along straight lines, not along crooked, curved, or winding lines. Moreover, such bodies will follow not every straight line lying between them and the surface of the earth, but only those select straight lines that are perpendicular to the surface of the earth and to the earth’s diameter.⁑ Also, celestial bodies move along circular lines rather than along straight lines or lines of other kinds. And when we examine natural motions, we will find that each of them takes place according to specific lines. So it is not impossible for sight to be constituted in such a way as to suffer the effects of light and color along a specific set of straight lines that alone intersect at its center and are perpendicular to its surface. Furthermore, it is granted by mathematicians that sight perceives visible objects along only those straight lines whose endpoints meet at the center of the eye, and there is no disagreement among them about this point.⁑ These lines are called »radial lines« by them.

6.44 ◉ Et cum hoc sit possibile, et forme lucis et coloris veniant ad visum et pertranseant per diafonitatem tunicarum visus, et visio non completur ex receptione istarum formarum nisi quando visus receperit istas ex verticationibus perpendicularium tantum, visus ergo non comprehendit luces et colores superficierum rerum visarum nisi ex formis venientibus ad ipsum ex superficiebus rerum visarum. Et non comprehendit istas formas nisi ex verticationibus linearum rectarum quarum extremitates concurrunt apud centrum visus tantum.

6.44 ◉ Since this is possible, and since forms of light and color reach the eye and pass through the transparency of the tunics of the eye, and since vision is achieved upon the reception of these forms only when the eye receives them along perpendicular lines, sight perceives the light and colors of the surfaces of visible objects only through forms reaching it from the surfaces of visible objects. Moreover, it perceives these forms only along those particular straight lines whose endpoints meet at the center of the eye.

6.45 ◉ Aggregemus modo ea que possunt aggregari ex omni quod diximus.

6.45 ◉ Let us now summarize what can be concluded from everything we have said.

6.46 ◉ Et dicamus quod visus sentit lucem et colorem que sunt in superficie rei vise ex forma extensa et luce et colore que sunt in superficie rei vise per corpus diafonum quod est medium inter visum et rem visam, et nichil comprehendit visus ex formis rerum visarum nisi ex verticationibus linearum rectarum extensarum inter rem visam et centrum visus tantum. Et declaratum est cum hoc quod hoc sit possibile et non impossibile.

6.46 ◉ And let us say that vision senses the light and color on the surface of a visible object through the form of both the light and color that extend from the surface of the visible object through the transparent medium that lies between the eye and the visible object, and sight perceives the forms of visible objects only along the straight lines that are extended between the visible object and the center of the eye. And along with this it has been shown that this is possible rather than impossible.⁑

6.47 ◉ Nos vero exponemus questionem, dicendo quod visio non potest esse nisi secundum hunc modum. Quoniam visus, quando senserit rem visam postquam non sentiebat ipsam, aliquid accidet ei quod non erat, et nichil accidet postquam non erat prius nisi per aliquam causam. Et invenimus quod, quando fuerit visus oppositus rei vise, sentiet ipsam; et cum auferetur ab eius oppositione, non sentiet ipsam, et cum revertetur ad oppositionem, revertetur sensus. Et similiter invenimus visum, quando senserit rem visam deinde clauserit palpebras, quod sensus destruetur; et cum aperit palpebras et res visa fuerit in oppositione, revertetur sensus. Sed causa est illud quod, quando destruetur, destruetur causatum; et quando revertitur, revertetur causatum. Causa ergo que facit contingere rem visam in visu est res visa apud oppositionem suam visui. Visus ergo non sentit rem visam nisi propter illud quod facit rem visam contingere in visu apud suam oppositionem visui.

6.47 ◉ But we will expound on the issue by saying that vision can only occur in this way. For when sight senses a visible object after having not sensed it, something that was not affecting it before now affects it, but nothing will happen later that was not in effect earlier except through some cause. And we find that when the eye faces a visible object, it will sense it; but when it is removed from that facing position, it will not sense it, whereas when it is brought back to the facing position, the sensation returns. Likewise, we find that when the eye senses a visible object and then [the viewer] closes his eyelids, the sensation ceases; but when he opens his eyelids while the visible object faces him, the sensation returns. Now a cause is such that, when it ceases to operate, what it causes ceases to exist; and when it is brought back to bear, what it causes comes back into existence. Therefore, what causes the visible object to have an effect on sight is the fact that the visible object faces the eye. Hence, sight does not sense a visible object unless the visible object creates an effect on it as it faces the eye.⁑

6.48 ◉ Et etiam visus non comprehendit rem visam nisi quando corpus quod est medium inter ea fuerit diafonum. Nam comprehensio visus de re visa ex posteriori aeris qui est medius inter eos non est propter humiditatem aeris sed propter diafonitatem eius, quoniam si medius fuerit inter visum et rem visam aliquis lapis aut aliud corpus diafonum quodcumque, etiam comprehendet tunc visus rem visam. Et erit comprehensio secundum diafonitatem corporis mediantis, et quanto corpus medium fuerit magis diafonum tanto magis erit sensus visus de illa re visa manifestior. Et similiter quando fuerit inter visum et rem visam aqua clara diafona, comprehendet visus rem visam a posteriori aque; et si illa aqua fuerit intincta aliqua tinctura forti ita quod destruatur diafonitas, quamvis remaneat in ea humiditas, tunc visus non comprehendet illam rem visam que est in aqua.

6.48 ◉ In addition, sight does not perceive a visible object unless the intervening medium is transparent. Now the visual perception of a visible object through the air that lies between eye and object is not due to the moisture in the air but, rather, to its transparency, for if some [transparent] stone or any other transparent body is interposed between the eye and the visible object, sight will still perceive the visible object. And the [clarity of] perception will depend upon the transparency of the intervening body, so that the more transparent the intervening body the clearer the visual sensation of the visible object. Likewise, when clear, transparent water intervenes between the eye and the visible object, sight will perceive a visible object through the water; but if that water is tinged with some strong dye so that its transparency is destroyed, then, even though the water’s moisture persists, sight will not perceive that visible object in the water.⁑

6.49 ◉ Declarabitur ergo ex istis dispositionibus quod visus non completur nisi per diafonitatem corporis medii, non per suam humiditatem. Illud ergo quod res visa operatur in visum apud suam oppositionem contra illum ex quo est sensus non completur nisi per diafonitatem corporis medii inter visum et rem visam. Lux ergo et color rei vise non comprehenditur a visu nisi ex aliquo quod sit ex illa luce et colore in visu, et illud non accidit ex colore et luce in visu nisi quando corpus medium inter visum et rem visam fuerit diafonum.

6.49 ◉ It will therefore be clear from these circumstances that sight is achieved only because of the transparency of the intervening medium, not because of its moisture. Hence, the effect that the visible object creates in sight when it faces what arouses sensation in it is realized only through the transparency of the medium between the eye and the visible object. The light and color of a visible object are therefore perceived by sight only by means of the effect of that light and color in the eye, and this effect is not created in the eye by color and light unless the medium between the eye and the visible object is transparent.

6.50 ◉ Diafonitas autem non appropriatur alicui ex eis que pendent ex luce et colore quo diversificetur a non diafonitate nisi quia forma lucis et coloris pertranseunt per diafonum, et non pertranseunt in non diafonum, et quia corpus diafonum recipit formam lucis et coloris et redit ipsam partibus oppositis luci et colori; corpus autem non diafonum non habet istam proprietatem. Et cum visus non sentit lucem et colorem que sunt in re visa nisi ex aliquo contingente ex luce et colore in visu, et illud non contingit in visu nisi quando corpus medium inter visum et rem visam fuerit diafonum, et corpus diafonum nullo appropriatur quo distinguatur a corpore non diafono ex eis que pendent a luce et colore nisi per receptionem formarum et colorum et reditionem eorum ad partes oppositas, et declaratum est quod, visus quando fuerit oppositus rei vise, forma lucis et coloris que sunt in re visa redentur visui et perveniunt in superficie sentientis, visus ergo non sentit lucem et colorem rei vise nisi ex forma extensa per corpus diafonum inter rem visam et visum et ex re quam facit contingere res visa in visu apud suam oppositionem illi mediante corpore diafono.

6.50 ◉ In terms of its essential relationship to light and color, transparency differs from opacity only insofar as the form of light and color passes through a transparent object, whereas it does not pass through one that is not transparent, and insofar as a transparent body receives the form of light and color and transmits it in [all] directions facing the light and color; a body that is not transparent, on the other hand, does not possess this quality. And since sight senses the light and color in a visible object by means solely of an effect created by them in the eye; and since that effect is created in the eye only when the medium between the eye and the visible object is transparent; and since a transparent body is distinguished from one that is not transparent solely by the fact that, in regard to its essential relationship to light and color, it is suited to the reception of forms and colors as well as to their transmission in facing directions; and since it has been shown that, when the eye faces a visible object, the form of the light and color in the visible object are transmitted into the eye and reach the surface of the sensing organ, sight senses the light and color of a visible object by means solely of a form extending through the transparent medium between the visible object and the eye, that form creating in the eye the effect of the visible object that faces it across the transparent medium.

6.51 ◉ Et licet nobis dicere quod corpus diafonum recipit a visu aliquid et redit ipsum rei vise, et per continuationem istius rei inter visum et rem visam evenit sensus. Et hec est opinio ponentium radios.

6.51 ◉ Now we might claim that the transparent medium receives something from the eye and transmits it to the visible object, so that sensation comes about from the extension of this thing between the eye and the visible object. This is the opinion of the proponents of [visual] rays.⁑

6.52 ◉ Ponatur ergo quod ita sit et quod radii exeant a visu et pertranseant per corpus diafonum pervenientes ad rem visam, et per istos radios sit sensus. Et cum ita fuerit sensus, quero per istos radios aut redetur visui aliquid aut non redetur. Si vero sensus fuerit per radios, et non redunt visui aliquid, visus nichil sentiet. Sed visus sentit rem visam, et cum sentit rem visam et non sentit nisi mediantibus radiis, isti ergo radii qui sentiunt rem visam redunt visui aliquid per quod visus sentit rem visam. Et cum radii redunt visui aliquid ex quo erit sensus visus rei vise, visus non sentiet lucem et colorem que sunt in re visa nisi ex aliquo veniente a luce et colore que sunt in re visa ad visum, et radii redunt illud. Secundum ergo omnes dispositiones non erit visus nisi per adventum alicuius rei vise a re visa, sive exierint a visu radii sive non.

6.52 ◉ Accordingly, let it be supposed that such is the case and that [visual] rays issue from the eye and pass through the transparent medium to reach the visible object, and [suppose] that [visual] sensation occurs by means of these [visual] rays. But if [visual] sensation occurs in this way, I ask whether something is transmitted back to the eyes through those [visual] rays or not. On the one hand, if [visual] sensation occurs by means of [visual] rays, but they transmit nothing back to the eye, then sight will perceive nothing. On the other hand, sight does sense the visible object, and if it senses the visible object but does so only by means of [visual] rays, then those [visual] rays that sense the visible object [must] transmit something back to the eye by means of which sight senses the visible object. Yet if the [visual] rays transmit something back to the eyes [and it is] by means of this [that] visual sensation of that visible object will occur, then sight will sense the light and color in the visible object by means solely of something coming from the light and color in the visible object to the eye, and the [visual] rays [must] transmit it. Under all conditions, then, sight will only occur by means of some visible property reaching [the eye] from the visible object, whether or not [visual] rays issue from the eye.

6.53 ◉ Et iam declaratum est quod visio non completur nisi per diafonitatem corporis medii inter visum et rem visam, et non completur quando fuerit medium inter eos corpus non diafonum. Et est manifestum quod corpus diafonum in nullo distinguitur a non diafono nisi secundum modum predictum. Et cum ita est, ut diximus, et fuit declaratum quod forma lucis et coloris que sunt in re visa perveniunt ad visum quando fuerit opposita visui, illud ergo quod venit ex re visa ad visum per quod visus comprehendit lucem et colores que sunt in re visa secundum omnem dispositionem non est nisi ista forma, sive exeant radii sive non.

6.53 ◉ Now it has already been shown that vision is achieved only through the transparency of the medium intervening between the eye and the visible object, and it is not achieved when the medium between them is not transparent.⁑ It is obvious, moreover, that a transparent body is distinguished from one that is not transparent in no way other than the aforementioned one. This being the case, as we have said, and since it has been shown that the form of the light and color in a visible object reaches the eye when it faces the eye, then what comes to the eye from the visible object to provide the means by which it perceives the light and color in the visible object, no matter the circumstance, is this very form [and this form] alone, whether [visual] rays issue [from the eye] or not.

6.54 ◉ Et iam declaratum est quod forme lucis et coloris semper generantur in aere et in omnibus corporibus diafonis, et semper extenduntur in aere et in corporibus diafonis ad partes oppositas, sive oculus fuerit presens sive non. Exitus ergo radiorum est superfluus et otiosus. Visus ergo non sentit lucem et colorem rei vise nisi a forma veniente a luce et colore que sunt in re visa.

6.54 ◉ And it has already been shown that forms of light and color are continually generated in air and in all [other] transparent bodies, and these forms continually extend through the air, as well as through [other] transparent bodies, in various directions, whether the eye is present or not.⁑ Hence, the extramission of [visual] rays is superfluous and useless.⁑ Accordingly, the eye senses the light and color of the visible object only by the form coming from the light and color in the visible object.

6.55 ◉ Et iam declaratum est quod forma cuiuslibet puncti rei vise oppositi visui pervenit ad visum secundum verticationes multas diversas, et quod visus non potest apprehendere formam rei vise secundum suam ordinationem in superficie rei vise nisi quando receptio formarum fuerit ex verticationibus linearum rectarum que sunt perpendiculares super superficiem visus et super superficiem membri sentientis, et quod linee recte perpendiculares non erunt super istas superficies nisi quando centrum istarum superficierum fuerit unum punctum, et quod hoc est possibile. Et cum hoc totum sit sicut dictum est, oportet quod centrum superficiei glacialis et centrum superficiei visus sint unum punctum. Visus ergo nichil potest comprehendere ex formis rerum visarum nisi ex verticationibus linearum rectarum quarum extremitates concurrunt apud hoc centrum tantum. Et hoc est quod promisimus ante declarare in hoc capitulo in precedenti sermone de forma visus, et iam declaratum est: scilicet quod centrum glacialis et centrum superficiei visus sunt idem punctum commune.

6.55 ◉ Furthermore, it has already been shown that the form of every point on a visible object facing the eye reaches the eye along several different lines and that sight can apprehend the form of the visible object according to its actual arrangement on the surface of the visible object only when the forms are received along straight lines that are perpendicular to the surface of the eye as well as to the surface of the sensing organ; [it has been shown] as well that [these] straight lines will not be perpendicular to [both of] these surfaces unless the centers of these surfaces form a single point and that this is possible. And since all this is true as claimed, the center of the surface of the glacialis and the center of the surface of the eye must lie at a single point. Sight therefore can perceive only those forms of visible objects [that reach the eye] along the straight lines whose endpoints meet at this center. And this is what, earlier in our discussion of the shape of the eye, we promised to show in this chapter, and this has now been demonstrated: i. e., that the center of the glacialis and the center of the surface of the eye form the same common point.⁑

6.56 ◉ Et cum hoc declaratum est, remanet ergo modo considerare opinionem ponentium radios et declarare quid sit ex ea falsum et quid verum. Dicamus ergo si visio sit ex re exeunte ex visu ad rem visam, illa res aut est corpus aut non est corpus. Si est corpus, quando nos aspexerimus celum et viderimus stellas que sunt in eo, oportet quod in illa hora exeat a nostro visu corpus, et impleat illud quod est inter celum et terram, et quod nichil diminuatur a visu; et hoc est falsum. Visio ergo non est per corpus exiens a visu ad rem visam. Et si illud quod exit a visu fuerit non corpus, illud non sentiet rem visam, sensus enim non est nisi in corporibus. Nichil ergo exit a visu ad rem visam sentiens rem illam.

6.56 ◉ Now that this has been demonstrated, it remains for us to consider the opinion of the proponents of [visual] rays and to show what is false and what is true about that opinion. Accordingly, we should say that if vision results from something passing from the eye to the visible object, then that thing is either corporeal or not. If it is corporeal, then when we look at the sky and see the stars in it, at that moment physical substance must stream from our eyes to fill the space between the heavens and the earth without the eye’s being diminished in any way; but this is illogical.⁑ Therefore, vision cannot be due to the extramission of some physical substance by the eye to the visible object. But if what is emitted from the eye is not corporeal, it will not feel the visible object, for sensation can only occur in bodies. Thus, nothing issues from the eye to the visible object to sense that object.

6.57 ◉ Et manifestum est quod visio est per visum. Et cum hoc est, et visus non comprehendit rem visam nisi quando exit ab eo aliquid ad rem visam, et illud quod exit non sentit rem illam visam, illud ergo quod exit a visu ad rem visam non redit ad visum aliquid quo visus comprehendit rem visam. Et hoc quod exit a visu non est sensibile sed opinabile, et nichil debet putari nisi per rationem. Ponentes autem radios opinantur hoc quia illi invenerunt quod visus comprehendit rem visam et inter eos est spatium; et magnum est hominibus quod sensus non est nisi per contactum, quare illi opinati sunt quod visio non sit nisi per aliquod exiens a visu ad rem visam, ita quod illud exiens sentiat rem visam in suo loco aut accipiat aliquid a re visa, et redet ipsum visui, et tunc sentiet illud visus.

6.57 ◉ And it is obvious that vision occurs through the eye. This being the case, if sight perceives a visible object only when something issues from the eye to the visible object but what issues [from the eye] does not sense the visible object, then what issues from the eye to the visible object transmits nothing back to the eye to serve as the means through which it can perceive the visible object. Also, the idea that something issues from the eye is based not on empirical evidence but on supposition, and nothing should be supposed unless dictated by logic. Yet the proponents of [visual] rays posit them because they have found that sight perceives a visible object when eye and object are spatially separated; but it is a cardinal precept among men that sensation cannot occur without [physical] contact, so the proponents of visual rays have concluded that vision only occurs through something issuing from the eye to the visible object and thereby sensing the visible object where it is or taking something from the visible object and transmitting it back to the eye, at which time the eye will sense it.⁑

6.58 ◉ Et cum non potest exire a visu corpus sentiens rem visam, et nichil sentit rem visam nisi sit corpus, non remansit opinari nisi quod illud quod a visu exit ad rem visam recipit a re visa aliquid et redit ipsum visui. Et cum declaratum est quod aer et corpora diafona recipiunt formam rei vise et redunt ipsam visui et omni corpori opposito rei vise, tunc illud quod opinatur quod redit visui aliquid ex re visa non est nisi aer et corpora diafona inter visum et rem visam. Et cum aer et corpora diafona redunt visui aliquid ex re visa, in quolibet tempore redunt et secundum omnes dispositiones quando visus fuerit oppositus rei vise sine indigentia alicuius rei exeuntis a visu. Ratio ergo que induxit ponentes radios ad dicendum radios esse est superflua, quoniam illud quod induxit eos ad dicendum quod radii essent est sua opinatio quia visio non potest compleri nisi per aliquod extensum inter visum et rem visam ut redat visui aliquid ex re visa. Et cum aer et corpora diafona faciant hoc sine indigentia alicuius rei exeuntis a visu, et sunt cum hoc extensa inter visum et rem visam, sine indigentia tunc ad apponendum aliam rem redentem visui aliquid de re visa nulla est opinio. Dicere ergo esse radios est nichil.

6.58 ◉ But since a sensitive body cannot issue from the eye to the visible object, and since only a body can sense a visible object, the only option left is to suppose that what issues from the eye to the visible object takes something from the visible object and transmits it to the eye. And since it has been shown that air and [other] transparent bodies receive the form of a visible object and transmit it to the eye as well as to every [other] body facing the visible object, what is assumed to transmit something from the visible object to the eye is nothing but the air or [other] transparent media intervening between the eye and the visible object. And since air and [other] transparent bodies transmit something from the visible object to the eye, they transmit it at any given moment and under all conditions when the eye faces the visible object [and they do so] without needing anything to issue from the eye. Thus, the reason that has led the proponents of [visual] rays to claim the existence of [such] rays is superfluous, because what has led them to claim that [visual] rays exist is their opinion that vision cannot be achieved except by something that extends from the eye to the visible object so as to transmit something back to the eye from the visible object. But since air and [other] transparent media fulfill this task without needing anything to issue from the eye, and, in addition, since they [already] extend between the eye and the visible object, then, since there is no need to suppose that something else transmits anything from the visible object to the eye, the opinion [of the proponents of visual rays] is pointless. Hence, the claim that [visual] rays exist is nullified.

6.59 ◉ Et etiam omnes mathematici dicentes esse radios non utuntur in demonstrationibus eorum nisi lineis ymaginatis tantum, et vocant ipsas lineas radiales. Et iam declaravimus nos quod visus nichil comprehendit ex rebus visis nisi ex verticationibus istarum linearum tantum. Opinio ergo opinantis quod linee radiales sunt ymaginate est opinio vera, et opinio opinantis quod aliquid exit a visu est opinio falsa. Et iam declaravimus quod hoc esse non affirmat eos nec ratio induxit.

6.59 ◉ Moreover, all the mathematicians who claim the existence of [such] rays use nothing but imaginary lines in their demonstrations, and they call them »radial lines.« But we have already shown that sight perceives visible objects along such lines alone. The opinion of those who suppose that radial lines are imaginary is thus true, whereas the opinion of those who suppose that anything issues from the eye is false.⁑ And we have now demonstrated that what actually obtains does not confirm [the existence of] visual rays, nor has reason led [us to accept] them.

6.60 ◉ Iam ergo declaratum est ex omnibus que diximus quod visus non sentit lucem et colorem que sunt in superficie rei vise nisi per formam extensam a superficie rei vise ad visum per corpus diafonum medium inter visum et rem visam, et quod visus nichil comprehendit ex formis nisi ex verticationibus linearum rectarum que ymaginantur extense inter rem visam et centrum visus tantum que sunt perpendiculares super omnes superficies tunicarum visus. Et hoc est quod voluimus declarare.

6.60 ◉ On the basis of everything we have said, then, it has now been demonstrated that the eye senses the light and color on the surface of a visible object by means solely of a form that extends from the surface of the visible object to the eye through a transparent medium intervening between the eye and the visible object, and [it has been demonstrated] that sight perceives only those forms [reaching the eye] along the straight lines that are conceived to extend between the visible object and the center of the eye, those lines alone being perpendicular to all the surfaces of the tunics of the eye. And this is what we wanted to demonstrate.

6.61 ◉ Ista ergo est qualitas visionis generaliter, quoniam visus non comprehendit ex re visa sensu spoliato nisi lucem et colorem que sunt in re visa tantum. Res autem residue quas comprehendit visus ex rebus visis, sicut figuram, et magnitudinem, et sibi similia, non comprehenduntur a visu spoliato sensu sed per rationem et signa. Et hoc nos declarabimus post in secundo tractatu post declarationem completam apud nostrum sermonem in distinctione rerum visibilium quas comprehendit visus. Et hoc quod declaravimus—scilicet qualitatem visionis—est conveniens opinioni verificantium mathesim et naturam. Et declaratum est ex hoc quod due secte dicunt verum et quod due opiniones sunt recte et convenientes, sed non completur altera earum nisi per alteram, nec potest esse visio nisi per illud quod aggregatur ex duabus.

6.61 ◉ This is therefore how vision takes place generally, because, in terms of naked sensation, sight perceives only the light and color that are in the visible object. The remaining characteristics of visible objects that sight perceives, e.g., shape, size, and the like, are perceived by sight not through naked sensation but through reason and defining features.⁑ And we shall show this later in the second book after we finish discussing the various visible properties that sight perceives. But what we have shown—namely, how vision takes place—conforms to the opinion of those who have verified it on mathematical grounds as well as [those who have verified it] on physical grounds. It has been shown therefore that both parties have something true to say and that both opinions are correct and compatible, but neither is wholly satisfactory without the other [to complement it], nor can vision be properly accounted for without drawing upon what both have to say.

6.62 ◉ Sensus ergo non est nisi ex forma et ex operatione forme in visum et ex passione visus ex forma, et visus est paratus ad patiendum ex ista forma secundum situm proprium, scilicet situm verticationum perpendicularium super suam superficiem. Natura autem visus non appropriatur ista proprietate nisi quia non distinguuntur visibilia nec ordinantur partes cuiuslibet eorum apud visum nisi quando sensus eius fuerit ex istis verticationibus tantum. Linee ergo radiales sunt linee ymaginabiles, et figuratur per eas qualitas situs super quam patitur visus ex forma.

6.62 ◉ Hence, [visual] sensation is due solely to the form and to the effect of the form on the eye as well as to the passion aroused in the eye by the form, and the eye is constituted in such a way as to be affected by this form according to a specific orientation, i.e., the orientation of perpendicular lines upon its surface. Moreover, it is in the nature of the eye to be so constituted only because individual visible objects would not be distinguished [by it], nor would the parts of any of them be properly arranged on the eye unless the sensation [aroused in it] occurred exclusively along those lines. Therefore, radial lines are imaginary lines, and they define the specific direction according to which the eye is affected by the form.

6.63 ◉ Et iam declaratum est quod, visus quando fuerit oppositus rei vise, figurabitur inter rem visam et centrum visus piramis cuius conus erit centrum visus et basis eius superficies rei vise. Et erit inter quodlibet punctum superficiei rei vise et inter centrum visus linea recta ymaginata perpendicularis super superficies tunicarum visus, et sic erit piramis continens omnes istas lineas. Et erit superficies glacialis secans istam piramidem, quoniam centrum visus quod est conus piramidis est a posteriori superficie glacialis; et cum aer qui est inter visum et rem visam fuerit continuus, erit forma extensa ab illa re visa secundum verticationem ipsius piramidis in aere quam distinguit ipsa piramis et in tunicis visus diafonis usque ad partem superficiei glacialis que distinguitur per istam piramidem. Et erit piramis continens omnes verticationes que sunt inter visum et rem visam ex quibus comprehendit visus formam rei vise, et erit forma ordinata in ista piramide sicut est ordinata in superficie rei vise et in ista parte superficiei glacialis.

6.63 ◉ And it has already been shown that when the eye faces a visible object, a cone will be formed between the visible object and the center of the eye, its vertex being the center of the eye and its base the surface of the visible object.⁑ And between any point on the surface of the visible object and the center of the eye there will be an imaginary straight line that is perpendicular to the surfaces of the tunics of the eye, and the cone will thus contain all such lines. And the [anterior] surface of the glacialis will cut this cone, for the center of the eye, which forms the vertex of the cone, lies behind the [anterior] surface of the glacialis; and if the air that intervenes between the eye and the visible object is continuous, the form will extend from the visible object along this cone through the air enclosed by it, as well as through the transparent tunics of the eye, to the area on the surface of the glacialis that is demarcated by that cone. And the cone will contain all the radial links between the eye and the visible object by means of which the eye perceives the form of that visible object, and that form will be arranged within this cone as it actually exists upon the surface of the visible object as well as upon the area on the surface of the glacialis [that is demarcated by the cone].⁑

6.64 ◉ Et iam declaratum est quod sensus non est nisi per glacialem. Sensus ergo visus ex lumine et colore que sunt in superficie rei vise non est nisi ex parte glacialis quam distinguit piramis figurata inter illam rem visam et centrum visus. Et iam predictum est quod in isto humore est aliquantule diafonitatis una pars et aliquantule spissitudinis, et propter hoc assimulatur glaciei. Quia ergo est in ea aliquantule diafonitatis, recipit formas, et pertranseunt in ea cum eo quod est ex ea de diafonitate; et quia in ea est aliquantule spissitudinis, prohibet formas a transitu in ea cum eo quod est ex ea de spissitudine. Et figuntur forme in eius superficie et corpore. Et similiter quodlibet corpus diafonum in quo est aliquid spissitudinis, quando super ipsum oritur lux, pertransibit in eo secundum id quod est in eo de diafonitate, et figitur lux in superficie eius secundum quod est in eo de spissitudine.

6.64 ◉ Furthermore, it has already been shown that [visual] sensation occurs only through the glacialis.⁑ Hence, the visual sensation of the light and color on the surface of a visible object occurs only at the area on the glacialis that is demarcated by the cone formed between that visible object and the center of the eye. And it has been said earlier that there is some measure of transparency and some measure of opacity in this humor and, accordingly, that it is like ice in appearance.⁑ Therefore, insofar as there is some transparency in it, it receives forms, and they pass through it by virtue of the transparency that is in it; but insofar as there is some opacity in it, it impedes the forms passing through it by virtue of the [modicum of] opacity it possesses. And the forms are [thereby] impressed on its surface and within its body.⁑ Likewise, when it shines upon a transparent body possessing some measure of opacity, light will pass through it on account of its transparency, but the light is impressed on its surface according to its opacity.

6.65 ◉ Et etiam glacialis est preparata recipere istas formas et ad sentiendum ipsas. Forme ergo pertranseunt in ea propter virtutem sensibilem recipientem.

6.65 ◉ Also, the glacialis is constituted to receive these forms and to sense them. The forms thus pass through it according to its capacity to receive them sensibly.

6.66 ◉ Et cum forma pervenit in superficie glacialis, operabitur in ea, et glacialis patietur ex ea, quoniam ex proprietate lucis est ut operetur in visu et ex proprietate visus est ut patiatur ex luce. Et ista operatio quam operatur lux in glaciali pertransit corpus glacialis secundum rectitudinem linearum radialium tantum, quoniam glacialis est preparata ad recipiendum formas lucis ex verticationibus linearum radialium. Et cum lux pertransit in corpus glacialis, color pertransit cum ea, color enim est admixtus cum luce. Et glacialis recipit istam operationem et istum pertransitum, et ex ista operatione et passione erit sensus glacialis ex formis rerum visarum que sunt in sua superficie. Et pertranseunt per totum suum corpus, et ex ordinatione partium forme in sua superficie et in suo toto corpore erit sensus eius ex ordinatione partium operantis.

6.66 ◉ And when the form reaches the surface of the glacialis, it will create an effect in it, and the glacialis will suffer that effect, because it is a property of light to affect the eye, and it is a property of the eye to be affected by light. And this effect that light creates in the glacialis passes through the body of the glacialis along straight, radial lines exclusively, for the glacialis is constituted to receive the forms of light along radial lines. And as the light passes through the body of the glacialis, color passes along with it, for color is mingled with light. For its part, the glacialis accepts this effect and its passage, and from this effect and the passion [aroused by it] the glacialis will sense the forms of the visible objects that are [incident] upon its surface. And those forms pass through its whole body, and from the arrangement of the parts of the form on its surface, as well as within its whole body, it will sense the arrangement of the parts of the [visible body] affecting it.⁑

6.67 ◉ Et ista operatio quam operatur lux in glacialem est ex genere doloris. Tamen quidam dolores sunt passibiles et non angustiatur membrum propter eos, et tales dolores non manifestantur sensui, nec iudicat dolens quod sit dolor. Et significatio super hoc est quod lux inducit dolorem, quia luces fortes angustiant visum et ledunt manifeste, sicut lux solis, quando aspiciens aspexerit corpus ipsius, et sicut lux solis reflexa a corporibus tersis ad visum, quoniam iste luces inducunt dolorem manifestum in visum. Et operatio omnis lucis in visum est ex uno genere, et non diversatur nisi secundum magis et minus. Et cum omnes sunt ex uno genere, et operatio fortiorum lucium est ex genere doloris, omnes ergo operationes lucium sunt de genere doloris, et non diversantur nisi secundum magis et minus. Et propter levitatem operationum lucium debilium temperatarum in visum, latet sensus eas inducere dolorem. Sensus ergo glacialis ex operatione lucis est de genere sensibilis dolorosi.

6.67 ◉ And the effect that light has upon the glacialis is in the form of pain. Now some pains can indeed be suffered without the [affected] organ’s being distressed by them, and such pains are not perceptible to sense, so the sufferer does not recognize them as pain. An indication of this fact is that light arouses pain insofar as strong light, such as sunlight, when a viewer stares at the sun itself, or sunlight, when it is reflected to the eye from polished bodies, distresses the eye and clearly hurts it, for such [strong] light arouses obvious pain in the eye. But the effect of all light upon the eye is of the same kind, varying only in intensity. And since these effects are all of the same kind, and since the effect of stronger light is in the form of [manifest] pain, then every effect of light is in the form of pain, varying only in intensity. And because of the lightness of the effects of weak and moderate light upon the eye, the [visual] sense fails to recognize them as pain. Hence, the sensation aroused in the glacialis by the effect of light is of the same kind as sensible pain.⁑

6.68 ◉ Deinde iste sensus qui cadit apud glacialem extenditur in nervo obtico et venit ad anterius cerebri, et illic est ultimus sensus et sentiens ultimum quod est virtus sensitiva que est in anteriori cerebri, et ista virtus comprehendet sensibilia. Visus autem non est nisi quoddam instrumentum istius virtutis, quoniam visus recipit formas rerum visarum et redit eas sentienti ultimo, et sentiens ultimum comprehendit illas formas et comprehendit ex eis res visibiles que sunt in eis. Et illa forma in superficie glacialis extenditur in corpore glacialis, deinde in corpus subtile quod est in concavo nervi quousque perveniat ad nervum communem. Et apud perventum forme ad nervum communem completur visio, et ex forma perveniente in nervo communi comprehendet ultimum sentiens formas rerum visarum.

6.68 ◉ After occurring at the glacialis, this sensation spreads through the hollow [optic] nerve and arrives at the front of the brain where sensation culminates and where the final sensor is located, this latter being the sensitive faculty at the front of the brain, and this faculty will perceive all sensibles. The eye, for its part, is nothing more than an instrument for this faculty, for the eye receives the forms of visible objects and transmits them to the final sensor, but the final sensor perceives those forms and perceives the visible properties possessed by them. And the form [impressed] on the surface of the glacialis extends through the body of the glacialis, then through the subtle flux [of visual spirit] in the hollow of the [optic] nerve until it reaches the common nerve. When the form reaches the common nerve, the visual process is complete, and from the form that arrives at the common nerve the final sensor will perceive the forms of visible objects.⁑

6.69 ◉ Sed aspiciens comprehendet res visas duobus oculis; et sic oportet ut forma rei vise perveniat in utroque visu, quare pervenient in visu ab una re visa due forme. Tamen aspiciens comprehendet unam rem visam, et hoc est quia due forme que perveniunt in duobus visibus ex uno viso, quando perveniunt ad nervum communem, concurrunt due forme, et superponetur una alii, et efficietur una forma. Et ex illa forma adunata ex duabus formis comprehendet ultimum sentiens formam illius visi.

6.69 ◉ However, the viewer will perceive visible objects with two eyes; thus, since the form of a visible object must reach both eyes, two forms will reach the visual faculty from a single visible object. Nevertheless, the viewer will perceive the visible object as single, and the reason is that, when the two forms reaching the two eyes from one visible object reach the common nerve, the two forms meet and are superimposed upon one another to make a single form. And it is from this form, which is united from the two [original] forms, that the final sensor will perceive the form of that visible object.⁑

6.70 ◉ Et significatio super hoc quod due forme que perveniunt in duobus oculis ab uno viso adunantur et efficiuntur una forma antequam comprehendat ipsam ultimus sensator et quod ultimus sensator non comprehendit formam nisi post adunationem duarum formarum est quod aspiciens, quando mutaverit situm sui oculi unius et alius fuerit immotus, et motus oculi mutati secundum situm fuerit ad anterius, videbit de re una opposita duas. Et si elevaverit unum oculum et cooperuerit unum oculum, non videbit nisi unum.

6.70 ◉ That the two forms reaching both eyes from a single visible object are united and made into a single form before the final sensor perceives it and that the final sensor perceives the form only after the two [original] forms are united is indicated by the fact that, when a viewer moves one of his eyes while the other remains immobile, and when the motion of the eye that is moved is in an upward direction, he will see a facing visible object doubled. If, however, he elevates the one eye while covering the other, he will only see [the object] as single.

6.71 ◉ Si ergo sentiens comprehendisset unum quia unum, deberet comprehendere ipsum semper unum; et si venissent ad ipsum semper due forme ab uno viso comprehenderet semper unum visum duo. Et cum ultimum sentiens non comprehendet visum nisi ex forma veniente ad ipsum et aliquando comprehendet unam rem visam duo et aliquando unum est signum quod id quod venit ad ipsum, quando comprehendit ipsum duo, veniunt due forme, et quando comprehendit unam rem visam unam, quod venit ad ipsum est forma una. Et cum in utraque dispositione perveniunt ab uno viso in duos oculos due forme, et illud quod reditur ultimo sentienti aliquando est due forme et aliquando una forma, et forma que reditur ultimo sentienti non reditur nisi a visu, tunc illud quod reditur ultimo sentienti ex duabus formis que perveniunt in duobus oculis ab uno viso, quando comprehendit ipsum unum, est una forma.

6.71 ◉ Thus, if the [final] sensor were to perceive [an object as] single [just] because it is single, then it ought to perceive it as single all the time; and if two forms were always to come to it from one visible object, then it would invariably perceive the single object doubled. But since the final sensor will only perceive the visible object through a form reaching it, the fact that it will sometimes perceive a single object as double and sometimes as single indicates that, when it perceives the object doubled, two forms reach it, whereas when it perceives the single visible object as single, a single form reaches it. In both cases, since two forms reach the two eyes from a single visible object, and since what is transmitted to the final sensor is sometimes two forms and sometimes a single form, and since the form that is transmitted to the final sensor is transmitted only by the eye, then, when it perceives a single object singly, what is transmitted to the final sensor from that object is a single form [arising] from the two forms reaching the two eyes from the single visible object.

6.72 ◉ Et cum ita est, due ergo forme predicte extenduntur a duobus oculis et concurrunt antequam comprehendat ipsas sentiens ultimus, et post concursum adinvicem comprehendet sentiens ultimus formam adunatam ex eis. Et due forme que perveniunt in duobus oculis ab uno viso, quando comprehendit ipsum duo, extenduntur a duobus oculis et non concurrunt, et perveniunt ad ultimum sentiens et sunt due forme.

6.72 ◉ Since this is the case, then the two aforementioned forms extend from the two eyes and meet before the final sensor perceives them, and it is after their juncture that the final sensor will perceive the form united from them. But when the final sensor perceives the two forms that reach the two eyes from a single object as double, those two forms extend from the two eyes but do not meet [in perfect superposition], so they reach the final sensor as two forms.

6.73 ◉ Et etiam comprehensio unius visi apparentis aliquando unum aliquando duo significat quod visio non est per oculum solummodo, quoniam si ita esset, apud comprehensionem unius visi apparentis comprehenderent duo oculi ex duabus formis provenientibus in eos unam et eamdem formam. Et si ita esset, comprehenderent semper ex duabus formis unam.

6.73 ◉ Moreover, the fact that a single visible object is sometimes perceived as single and sometimes as double indicates that vision is not due to the eye alone, for it it were, then at the moment of its perception the two eyes would perceive the two forms reaching them from a single object as one and the same form. And if that were the case, then they would always perceive one form from those two.

6.74 ◉ Et cum unum visum comprehendatur aliquando unum et aliquando duo, et in utraque dispositione sint in duobus oculis due forme significat quod illic est alius sentiens preter duos oculos apud quem perveniunt ab uno viso quando comprehenduntur per unum due forme unum et apud quem comprehenduntur due forme quando comprehenduntur due, et quod sensus non completur nisi per illud sentiens tantum, non per oculum tantum.

6.74 ◉ And the fact that a single visible object is sometimes perceived as single and sometimes as double, while in either case two forms are [impressed] in the two eyes, indicates that, besides the two eyes, there is some sensitive agent according to which the two forms extending from a single object that is perceived singly are perceived as one and according to which the two forms are perceived as two when the object is perceived as double, which indicates that [visual] sensation is fully achieved only by that sensitive agent, not by the eyes alone.

6.75 ◉ Et etiam sensus non extenditur a membris ad ultimum sentiens nisi in nervis continuatis membris et cerebro. Due ergo forme extenduntur ab oculo in nervo extenso inter oculum et cerebrum quousque perveniant ad ultimum sentiens. Iste ergo due forme extenduntur a duobus oculis et concurrunt in loco concursus duorum nervorum.

6.75 ◉ In addition, [visual] sensation extends from the [sensing] organs to the final sensor only through the nerves that link those organs and the brain. Therefore, the two forms pass from the eye through the nerve that extends between the eye and the brain until it reaches the final sensor. These two forms thus pass from the two eyes and meet where the two nerves join.

6.76 ◉ Et significatio manifesta quod forme rerum visarum extenduntur in concavo nervi, et perveniunt ad ultimum sentiens, et post perventum completur visio est quod, quando fuerit opilatio in isto nervo, destruetur visio, et cum destruetur opilatio, revertetur visio. Et ars medicinalis testatur hoc.

6.76 ◉ And clear evidence that the forms of visible objects extend through the hollow of the nerve to reach the final sensor and that vision is achieved [only] after [their] arrival there is that, when there is some obstruction in this nerve, vision is destroyed, but when the obstruction is removed, vision is restored. And medical science testifies to this fact.⁑

6.77 ◉ Quare vero aliquando concurrunt due forme et aliquando non est quia, quando situs duorum oculorum fuerit naturalis, erit situs eorum ab uno viso situs consimilis, et sic perveniet forma unius visi in duo loca consimilis situs. Et cum fuerit declinans situs unius oculorum, diversabitur situs oculorum ab illo viso, et sic perveniunt due forme illius visi diversi situs. Et iam predictum est in forma oculi quod situs nervi communis a duobus oculis est situs consimilis, et sic erit situs duorum locorum consimilis situs a duobus oculis ab eodem loco nervi communis situs consimilis, et ex duobus nervis concavis fit unus in quo uniuntur due forme visus.

6.77 ◉ Now the reason that the two forms sometimes join and sometimes do not is that, when the two eyes are in their natural position, they will be similarly oriented with respect to the single visible object, and thus the form of the single object will reach two places [on the surfaces of the two eyes] that are similarly oriented. However, when one eye is displaced, the orientation of the eyes will differ with respect to that visible object, and thus the two forms of that object will reach [two places on the surfaces of the two eyes] with different orientations.⁑ But it has already been mentioned in [the section on] the structure of the eye that the common nerve is similarly oriented with respect to the two eyes,⁑ and so two spots at corresponding locations on the two eyes will be similarly oriented with respect to the same location in the common nerve, and the two hollows of the nerve are joined to form a single place where the two forms of the visible object are united.

6.80 ◉ Et licet dicere quod forme pervenientes in oculo non perveniunt ad nervum communem, sed sensus extendetur ab oculo ad nervum communem sicut extenditur sensus doloris et sensus tactus, et tunc comprehendit ultimum sentiens illud sensibile.

6.80 ◉ We might claim that the forms arriving at the eye do not reach the common nerve, but that the sensible effect [of those forms] will instead extend from the eye to the common nerve, just as the sense of pain and the sense of touch [extend through the nerves], and that at this time the final sensor perceives that sensible effect.

6.81 ◉ Et nos dicemus quod sensus ipse veniens ad oculum pervenit ad nervum communem omnino; tamen sensus qui pervenit ad oculum non est sensus doloris tantum, sed est sensus operationis de genere doloris, et sensus lucis et coloris, et sensus ordinationis partium visi. Sensus autem diversitatis coloris et ordinationis partium visi non est de genere doloris. Et nos declarabimus post quomodo erit sensus visus ex omnibus rebus istis. Sensus ergo perveniens in nervo communi est sensus lucis et coloris et ordinationis, et illud a quo comprehendit sentiens ultimum lucem et colorem est aliqua forma.

6.81 ◉ And we shall respond that this sensation arising in the eye does indeed reach the common nerve; still, the sensation arising in the eye is not a sensation of pain alone; it is a sensation of the effect of a kind of pain along with a sensation of light and color as well as of the arrangement of the parts of the visible object. However, the sensation of different colors and of the arrangement of the parts of a visible object is not of the nature of pain. And we shall show later how the visual sensation of all these qualities occurs.⁑ Therefore, the sensation that reaches the common nerve includes the sensation of light and color and of the arrangement [of parts], and it is by means of some form that the final sensor perceives light and color.

6.83 ◉ Et remanet modo dicere questionem que est: Quando forme lucis et coloris extenduntur in aere et in corporibus diafonis et perveniunt ad visum, et aer et corpora diafona recipiunt omnes colores, et forme cuiuslibet lucis que sunt presentes in eodem tempore extenduntur in eodem tempore et in eodem aere, et perveniunt ad unum oculum et pertranseunt diafonitatem tunicarum visus, quare oportet ut admisceantur isti colores et lux in aere et in corporibus diafonis et perveniant ad visum omnia mixta, et sic non distinguentur a visu colores rerum visarum. Et si ita est, sensus ergo visus non potest esse ex istis formis.

6.83 ◉ We are now left to address the following issue: When the forms of light and color extend through air as well as through [other] transparent bodies to reach the eye, since air and [other] transparent bodies accept all colors, and since the forms of any light that are present at the same time extend through the same air at the same time and pass through the transparency of the tunics of the eye when they reach a single eye, then these colors and light ought to mingle in the air and in the [other] transparent bodies and arrive at the eye completely mixed, and so the colors of visible objects will not be [individually] discerned by sight. And if this is the case, then visual sensation cannot be due to these forms.

6.84 ◉ Dicamus ergo quod aer et corpora diafona non immutantur a coloribus nec alterantur ab eis alteratione fixa, sed proprietas coloris et lucis est quod forme eorum extenduntur secundum verticationes rectas, et ex proprietate corporis diafoni est quod non prohibet formas lucis et coloris transire per suam diafonitatem. Et illud non recipit formas istas nisi receptione ad redendum, non receptione ut alteretur. Et declaratum est quod forme lucis et coloris non extenduntur in aere nisi secundum lineas rectas. Forme ergo lucis et coloris que sunt in corporibus presentibus simul in eodem aere extenduntur secundum lineas rectas, et erunt ille linee super quas extenduntur forme diverse quedam equidistantes, et quedam secantes se, et quedam diversi situs; et quelibet verticatio earum est distincta per corpus a quo descenditur forma super illam verticationem. Forme ergo extense a corporibus diversis in eodem aere extenditur quelibet earum super suam verticationem et pertransit ad formas oppositas.

6.84 ◉ Let us reply, accordingly, that air and [other] transparent bodies are neither transformed nor altered by colors in a permanent way; rather, it is in the nature of color and light that their forms extend along straight lines, and it is in the nature of a transparent body that it not prevent the forms of light and color from passing through its transparency. And it accepts these forms only to transmit them, not to be transformed upon accepting them. Furthermore, it has been shown that the forms of light and color extend through air only along straight lines.⁑ Therefore, the forms of the light and color in bodies that occupy the same air at the same time extend along straight lines, but [some of] those lines along which the different forms extend will be parallel, some will intersect, and others will have various [other] orientations; but each of these lines is distinguished by the body from which the form radiates along that line. Thus, each of the forms extending from different bodies through the same air extends along its own line and passes through to facing forms.

6.85 ◉ Et significatio quod luces et colores non admiscentur in aere nec in corporibus diafonis est quod, quando in uno loco fuerint multe candele in locis diversis distinctis, et fuerint omnes opposite uni foramini pertranseunti ad locum obscurum, et fuerit in oppositione illius foraminis in obscuro loco paries aut corpus non diafonum, luces illarum candelarum apparent super corpus vel super illum parietem distincte secundum numerum candelarum illarum; et quelibet illarum apparet opposita uni candele secundum lineam rectam transeuntem per foramen. Et si cooperiatur una candela, destruetur lux opposita illi candele tantum, et si auferatur coopertorium revertetur lux.

6.85 ◉ Moreover, evidence that light and colors do not mingle in air or in [other] transparent bodies is [found in] the fact that, when several candles are at various distinct locations in the same area, and when they all face a window that opens into a dark recess, and when there is a white wall or [other white] opaque body in the dark recess facing that window, the [individual] lights of those candles appear individually upon that body or wall according to the number of those candles; and each of those [spots of light] appears directly opposite one [particular] candle along a straight line passing through the window. Moreover, if one candle is shielded, only the light opposite that candle will be extinguished, but if the shielding body is lifted, the light will return.

6.86 ◉ Et hoc poterit omni hora probari.

6.86 ◉ And this can be tried anytime.

6.87 ◉ Et si luces admiscerentur cum aere, admiscerentur in aere foraminis; et debent transire admixte et non distinguentur postea. Et nos non invenimus ita. Luces ergo non admiscentur in aere, sed quelibet illarum extenditur super verticationes rectas; et ille verticationes sunt equidistantes, et secantes se, et diversi situs. Et forma cuiuslibet lucis extenditur super omnes verticationes que possunt extendi in illo aere ab illa, et cum hoc non admiscentur in aere, nec aer tingitur per eas; sed pertranseunt per suam diafonitatem tantum, et aer cum hoc non admittit suam formam.

6.87 ◉ In addition, if the lights were to mingle with the air, then they would mix in the air contained by the window; they ought, then, to pass through mixed so as not to be [individually] discerned afterward. But we do not find this to be the case. Therefore, the lights do not mix in air; instead, each of them extends along straight lines; and those lines are parallel, or they intersect, or they have various [other] orientations. And the form of each light-source radiates along all the [straight] lines that can be extended from it through the air, and in accord with this [the resulting forms of light] do not mingle in the air, nor is the air tinted by them; rather, they merely pass through its transparency, and the air does not thereby become transformed.

6.89 ◉ Et quod diximus de luce et colore, et quod de aere, intelligendum est in omnibus corporibus diafonis et tunicis visus diafonis.

6.89 ◉ And what we have said about light and color, as well as about the air, should be understood [to apply] to all transparent bodies, including the transparent tunics of the eye.⁑

6.90 ◉ Membrum vero sentiens, scilicet glacialis, non recipit formam lucis et coloris sicut recipit aer et alia diafona non sentientia, sed secundum modum diversum ab illo modo, quoniam istud membrum est preparatum ad recipiendum istam formam. Recipit ergo istam in quantum est sentiens et in quantum est diafonum. Et iam declaratum est quod passio eius ex ista forma est ex genere doloris. Qualitas ergo receptionis eius ab ista forma est diversa a qualitate receptionis corporum diafonorum non sentientium. Sed tamen istud membrum cum sua receptione ab ista forma in quantum est sentiens et cum sua alteratione vel mutatione, non tingitur per istam formam sua tinctura, nec remanent forme coloris et lucis post recessum eius ab earum oppositione vel recessum earum.

6.90 ◉ However, the sensitive organ [of the eye], i.e., the glacialis, does not receive the form of light and color in the same way as air and other insensitive transparent bodies, but in a different way from that, for this organ is constituted for the [sensitive] reception of that form. Therefore, it receives the form both as a sensitive body and as a transparent body. And it has already been shown that the effect aroused in it by this form is a kind of pain. Thus, the way it receives this form is different from the way insensitive transparent bodies receive them. Nevertheless, although it receives this form as a sensitive body and is thereby altered or transformed, this organ is not tinted by the color of this form,⁑ nor do the forms of color and light persist in it after it ceases to face them or they cease to face it.

6.91 ◉ Et potest contradici huic sermoni, scilicet dicendo quoniam iam predictum est quod colores fortes scintillantes super quos oriantur luces fortes operantur in oculo, et remanent sue alterationes in visu post recessum, et remanent forme coloris in oculo tempore aliquanto; et quodcumque comprehenderit visus post hoc erit admixtum cum illis coloribus. Et hoc est manifestum, et non dubitatur. Et cum ita est, visus ergo tingitur a colore et luce, et sequitur ut corpora diafona tingantur a coloribus et lucibus.

6.91 ◉ But this point can be countered with the following argument: It has already been maintained [not only] that intense and bright colors upon which strong light shines create an effect in the eye, but [also that] the changes they cause in the eye persist after they are removed, and the forms of the color persist in the eye for some time; moreover, whatever the eye perceives afterward will be mingled with those colors.⁑ This is clear and indubitable. And since this is so, then the eye must be tinted by color and light, so it follows that [all] transparent bodies are tinted by colors and light.

6.92 ◉ Et nos dicemus respondendo quod hoc ipsum significat quod visus non tingitur a colore et luce, nec remanent in eo alterationes coloris et lucis, quoniam iste alterationes quas diximus non accidunt nisi extranea fortitudine lucis et coloris. Et manifestum est quod iste alterationes non remanent in visu nisi modico tempore et post auferuntur, et tunc debiles immutationes non remanet aliquid. Tunc ergo visus non tingitur ab istis alterationibus alteratione fixa, nec remanent in eo post recessum. Et ex hoc declarabitur quod luces et colores operantur in visum, nec remanent alterationes eorum post recessum, nec parvo tempore. Glacialis ergo alteratur a luce et coloribus tantum quod sentit, deinde aufertur immutatio post recessum. Alteratio ergo eius a colore et luce est necessaria, sed natura non fixa.

6.92 ◉ In response we shall say that this very phenomenon indicates that the eye is not tinted by color and light and that the alterations caused by color and light do not persist in it, for these alterations that we have mentioned only happen because of an excess in the intensity of light and color. And it is clear that these alterations persist in the eye only for a short time and disappear afterward, whereas weak alterations do not persist at all. The eye, therefore, is not tinted by these alterations in a permanent way, nor do they persist in it after they are removed. Accordingly, it will be evident that [moderate] light and colors affect the eye, but that after they are removed the alterations they cause do not persist even for a short time. Hence, the glacialis is altered by light and colors only [at the time] it senses [them], but then the effect disappears after they are removed. It is therefore requisite that it be altered by color and light, but not in a permanent way.

6.93 ◉ Et etiam visus est preparatus ad patiendum colores et luces et ad sentiendum eas, et cum hoc non remanet in eo alteratio. Et aer, et corpora diafona, et tunice diafone anteriores glacialis non sunt preparate ad patiendum lucem et colorem et sentiendum ea, et non sunt preparate nisi ad redendum luces et colores tantum.

6.93 ◉ Moreover, the eye is constituted to suffer the effect of colors and light and to feel them, but the resulting alteration does not thereby persist in it. On the other hand, air, [other] transparent bodies, and the transparent tunics of the eye in front of the glacialis are not constituted to suffer the effect of light and color and feel them, nor are they constituted to do anything but transmit light and colors.⁑

6.94 ◉ Iam ergo declaratum est quod visus non tingitur ex coloribus et formis lucis tinctura fixa. Et declaratum est quod forme lucis et coloris non admiscentur in aere et in corporibus diafonis et quod visus multi comprehendunt ipsos in aere et in eodem tempore; et quilibet eorum comprehendit ipsos secundum piramidem que distinguit inter ipsam et centrum visus.

6.94 ◉ It has therefore now been shown that the eye is not tinted by colors and the forms of light in a permanent way. It has also been shown that the forms of light and color do not mingle in air or in [other] transparent bodies but that the eye perceives many of them at the same time through the [same] air; and each of the eyes perceives them according to the cone that is formed between the visible object and the center of the sight.

6.95 ◉ Quare vero non apparent omnes forme omnium colorum super omnia corpora illa, sed quedam apparent et quedam non, non est nisi quando color fuerit fortis, et lux que est in colore fuerit fortis, et lux que est in corpore super quod apparet forma coloris debilis? Et hoc pertinet ad visum, quoniam iste forme non oriuntur super corpora opposita illis sed super corpora illuminata cum quolibet lumine colorato. Quoniam forma lucis et coloris eius semper oriuntur super omnia corpora opposita illis quorum remotio non est extranea multa fortis longa. In lucibus vero hoc manifestatur, quoniam, quando fuerit experimentatum omne corpus illuminatum quolibet lumine (ita quod non fuerit lux valde debilis), et fuerit experimentatum secundum modos quos declaravimusscilicet ut sit positum in sua oppositione corpus album, et illud corpus sit in loco obscuro, et fuerit inter corpus illuminatum et illum locum obscurum foramen strictumquoniam super illud corpus tunc apparebit lux. Colores autem non apparebunt nisi secundum modum proprium, quoniam declaratum est per inductionem quod forme colorum semper sunt debiliores ipsis coloribus, et quanto forme fuerint magis remote a suo principio tanto erunt debiliores.

6.95 ◉ But why is it that not all the forms of all the colors appear on all those bodies [upon which they shine], but that some appear and some do not, depending on whether the color is intense, or the light that illuminates the color is intense, or the illumination of the body upon which the form appears is faint? The eye is responsible for this, because these forms [that do not appear] are not [just] shining upon bodies that face them but upon bodies that are illuminated by some colored light. For the form of any body’s light and color continually shines upon all facing bodies when they do not lie too far away. As far as light is concerned, in fact, this is obvious, for, when any body that is somehow illuminated is tried (as long as the illumination is not very weak), and when the trial is carried out as we have described—i.e., when a white body is placed opposite it within a dark recess, and when there is a narrow opening between the illuminated object and that dark recess—[it is obvious] that the light will then appear upon that body.⁑ On the other hand, colors will appear only under the proper conditions, for it has been shown by induction that the forms of colors are always weaker than the colors themselves, and the farther the forms are from their source, the weaker they will be.⁑

6.96 ◉ Et declaratum est etiam per inductionem quod fortes colores, quando fuerint in locis obscuris et fuerint luces que sunt super ipsos valde debiles, illi colores apparebunt obscuri, et non distinguentur visui. Et quando fuerint in locis illuminatis, et fuerit lux que est super eum fortis, apparebunt colores et distinguentur visui.

6.96 ◉ It has also been shown by induction that, when intense colors are situated in dark places and the light that shines upon them is very weak, those colors will appear dark and will not be [properly] discerned by sight. But when they are situated in well-lit places and the light shining upon them is strong, the colors will appear and will be [properly] discerned by sight.⁑

6.97 ◉ Et declaratum est etiam per inductionem quod, quando lux fortis fuerit super formas colorum apparentes super corpora opposita illis, latebunt visum, et non apparebunt nisi quando lux non fuerit fortis vel remota.

6.97 ◉ Furthermore, it has been shown by induction that, when intense light shines upon the forms of colors appearing on bodies facing them, those colors will disappear from sight, and they will only appear when the light is not intense or [its source] is far away.⁑

6.98 ◉ Et etiam declaratum est quod, quando lux fuerit fortis et pervenerit ad visum, prohibebit ipsum ab apprehensione rerum visarum non apparentium in se multum oppositarum illi tunc.

6.98 ◉ It has also been shown that, when intense light shines on the eye, it will prevent it from seeing many visible objects that face it at that time but are not visible by themselves.⁑

6.99 ◉ Et etiam est declaratum quod visus non comprehendit colores nisi ex forma veniente ad ipsum ex illo colore et quod comprehensio eius erit secundum verticationes proprias. Quando ergo inspiciens aspexerit corpus densum super quod oriebatur forma coloris, non comprehendet illam formam nisi ex forma secunda veniente ad ipsum ex illa forma. Et ista forma secunda est debilior prima forma que est super illud corpus, et prima forma est debilior ipso colore. Et visus non comprehendit ipsum corpus densum super quod apparet forma nisi quando in ea apparet aliqua lux, sive lux veniens cum forma coloris super ipsum orientis, sive illa lux cum alia. Forma ergo secunda que venit ad visum ex prima forma coloris venit ad ipsum cum forma lucis que est in illo corpore denso. Et color illius corporis densi super quod est forma comprehendetur a visu etiam in illa dispositione. Forma ergo eius coloris venit ad visum cum forma secunda veniente ad ipsum ex forma coloris que est super ipsum, et forma coloris istius corporis que venit ad visum in illa dispositione est prima forma. Visus autem non comprehendit illud quod comprehendit nisi ex verticationibus propriis, et verticatio propria que est inter ipsum et corpus densum secundum quam comprehendit formam coloris illius corporis densi est eadem cum verticatione secundum quam comprehendit formam secundam venientem ex forma coloris orientis super illud corpus, quoniam illa forma est in superficie illius corporis. Visus ergo comprehendit ipsam ex verticationibus que sunt inter ipsum et illud corpus, et ipse comprehendit colorem illius corporis ex verticationibus que sunt inter ipsum et illud corpus. Et similiter comprehendit visus lucem que est in illo corpore ex illis eisdem verticationibus. Tres ergo forme venientes ex illo colore ad visum comprehenduntur a visu ex eadem verticatione.

6.99 ◉ It has been shown as well that the eye does not perceive colors except by means of a form reaching it from that color and that it will be perceived along the appropriate [radial] lines.⁑ Therefore, when a viewer looks at an opaque object upon which the form of the color has shone, he will perceive that form by means only of a secondary form reaching him from that form [shining on the object]. But this secondary form is weaker than the primary form [shining] on that body, whereas that primary form is weaker than the color itself [in the source-object]. Now sight does not perceive the opaque body upon which the form appears unless some light appears in it, whether it be the light that accompanies the form of the color shining on it or that light along with some other light. Thus, the secondary form that reaches the eye from the primary form comes to the eye along with the form of the light in that opaque body. But the color of that opaque body upon which the form lies will also be perceived by sight in that situation. Hence, the form of its color arrives at the eye along with the secondary form reaching it from the form of the color that shines upon it, but the form of the color of this body that reaches the eye in this situation is a primary form. The eye, moreover, perceives what it perceives only along specific [radial] lines, and the specific [radial] line between it and the opaque body along which it perceives the form of the color of that opaque body is the same as the [radial] line along which it perceives the secondary form coming [to it] from the form of color shining upon that body, for that form [too] lies on the surface of that body. Therefore, the eye perceives this form along the [radial] lines that lie between it and that body, and it perceives the color of that body along the [radial] lines that lie between it and that body. Likewise, the eye perceives the light in that body along these same [radial] lines. Hence, three forms of that color reaching the eye are perceived by the eye along the same [radial] line.⁑

6.100 ◉ Et cum hoc est, comprehenduntur admixte, et forme secunde que veniunt ad visum ex forma coloris que sunt super corpus oppositum illi comprehendentur a visu semper admixte cum forma coloris illius corporis et forma lucis eius. Ipsum ergo comprehendit ex congregatione duorum colorum formam diversam a forma cuiuslibet earum. Si ergo corpus densum super quod est forma habuerit fortem colorem, erit forma eius que venit ad visum fortis, et est prima forma, et est admixta cum secunda forma que venit ad ipsum ex forma coloris orientis super illud corpus. Et ista forma est debilis, quare non apparet visui quoniam, quando cum colore debili fuerit admixtus color fortis, vincet color fortis debilem. Et similiter inveniuntur semper colores et tincture quando admiscentur adinvicem. Forma vero coloris non latet quando lux que est super ipsam fuerit fortis nisi quia forma secunda venit ad visum cum forma lucis fortis et cum albedine corporis.

6.100 ◉ And since this is so, they are perceived mingled together, and the secondary forms that reach the eye from the form of color that shines upon the body facing it will always be perceived by the eye mingled with the form of the color of that body, as well as with the form of its light. The eye thus perceives a form derived from the two colors [and] that [form] is different from the form of either of them. If, then, the opaque body upon which the form [shines] is of a bright color, the form it conveys to the eye will be bright, and it is a primary form, and it is mingled with the secondary form that reaches the eye from the form of the color shining upon that body. But this form is weak, so it is not apparent to sight, because, when a bright color is mingled with a faint color, the bright color overwhelms the faint one. And the same things are invariably found [to obtain] in the case of colors and dyes when they are mixed together. However, the form of the color is invisible when the light that shines on it is intense only because the secondary form reaches the eye along with the form of intense light as well as with the whiteness of the body.

6.101 ◉ Et iam declaratum est quod lux fortis, quando venit ad visum, prohibet visum a comprehensione formarum debilium. Quando ergo veniet ad visum lux fortis cum albedine corporis supra quod est, prohibebit ipsum a comprehensione secunde forme debilis que venit ad ipsum cum ea. Et si corpus super quod est forma coloris fuerit album, et lux que est super ipsum fuerit debilis, et forma etiam coloris que est super ipsum fuerit debilis, tunc forma lucis que est in illo corpore, quamvis sit debilis, cum albedine corporis forte vincet formam coloris que est valde debilis. Et cum venerit ad visum, non distinguetur illa forma visui. Et si corpus super quod est lux fuerit album, et color cuius forma oritur super ipsum fuerit niger aut obscurus, non eclipsabitur illa forma nisi ab albedine illius corporis tantum; et erit quasi umbra, et comprehendet visus illud corpus non valde album, sicut comprehendet corpus album in umbra, quare non distinguetur ab eo forma.

6.101 ◉ Now it has already been shown that when intense light shines on the eye, it prevents the eye from perceiving weak forms.⁑ Therefore, when intense light reaches the eye along with the whiteness of the body upon which it shines, it will prevent the eye from perceiving the weak secondary form that reaches it along with that light. On the other hand, if the body upon which the form of the color shines is white, but if the light that shines upon it is weak and the form of the color that shines upon it is also weak, then, even though it is weak, the form of the light in that body, along with the body’s whiteness, will overwhelm the form of the color, which is very weak. So when it reaches the eye, that form will not be [properly] discerned by the eye. If, however, the body upon which the light shines is white and the color whose form shines upon it is black or dark, that form will be outshone only by the whiteness of that body; so it will appear as shadow, and the eye will perceive that body as not very white, just as it will perceive a white body in shadow, so its form will not be [properly] discerned by it.

6.102 ◉ Et omne hoc erit ita quando lux que est in corpore colorato fuerit fortis, et forma que oritur ab eo super corpus oppositum fuerit albedinis debilis. Si autem lux que est in corpore colorato fuerit debilis, tunc forma que exit ex eo super corpus oppositum erit obscura, et erit apud visum sicuti colores quos comprehendit in locis obscuris in quo est lux debilis valde et quasi colores corporum diafonorum super que oritur lux debilis. Forme ergo colorum que sunt in corporibus coloratis, quando lux que est super ipsas fuerit debilis, quando oriuntur super corpora opposita sibi, non erunt nisi umbre tantum quoad sensum visus. Et si corpus oppositum colori quod est huiusmodi fuerit in loco obscuro, nichil apparebit super ipsum propter suam obscuritatem et obscuritatem forme venientis ad ipsum. Et si corpus oppositum isti colori fuerit in illuminato loco, et fuerit super ipsum lux preter lucem illius forme, et fuerit illud corpus illuminatum, apparebit color eius super illam formam; et apparebit visui color istius corporis, et non apparebit forma, quoniam est sicut umbra, et non distinguetur visui ista diminutio. Et si istud corpus super quod est forma fuerit album et cum hoc fuerit illuminatum cum alio lumine preter lumen forme, tunc forma eclipsabit albedinem istius corporis et lucem eius tantum propter suam obscuritatem, sicut faciunt umbre in corporibus albis.

6.102 ◉ All of this will obtain when the light that illuminates the colored body is intense and the whiteness of the form that shines from it upon the facing body is dull. If, however, the light in the colored body is weak, then the form that shines from it upon the facing body will be dark, and it will appear to the eye just like the colors it perceives in dark, poorly lit locations or the colors of transparent bodies upon which weak light shines. Hence, when the light that shines upon colored bodies is feeble and when the forms of their colors shine on facing bodies, they will only appear as shadows as far as visual sensation is concerned. And if such a body facing the color lies in a dark location, none of the color will appear on it on account of its darkness and the darkness of the form shining on it. But if the body facing this color lies in an illuminated location and there is light other than the light of its form shining on it, and if this body is illuminated, then its color will appear superimposed upon that form; and the color of that body will appear to the eye but not the form, because it acts just like a shadow, and its shadowing effect will not be [properly] discerned by the eye. However, if that body upon which the form shines is white and, moreover, is illuminated by some light other than the form’s light, then, on account of its darkness, the form will merely dim the body’s whiteness and luminosity in much the same way as shadows are cast on white objects.

6.103 ◉ Et forme que sunt huiusmodi tantum comprehendentur a visu super corpora opposita coloribus.

6.103 ◉ And forms of this sort will only be perceived by the eye on bodies facing colors.

6.104 ◉ Visus ergo non comprehendit formam coloris super corpus oppositum colori nisi quando forma secunda veniens ad ipsum ex forma coloris fuerit fortior et magis vincens prima forma venienti ad ipsum cum ea ex luce et colore que sunt in corpore super quod est forma. Et iste modus est valde rarus, et propter hoc raro apparet huiusmodi forma, et non apparet ex ea nisi illud quod est ex coloribus fortibus scintillantibus, et quando lux que est super ipsos colores fuerit fortis, et quando iste forme fuerint super corpora opposite albedinis, et quando lux que est super ista corpora fuerit debilis in respectu illarum formarum. Et quod non est huiusmodi non apparet.

6.104 ◉ Therefore the eye does not perceive the form of a color on a body facing that color except when the secondary form reaching it from the form of its color is more intense and more overwhelming than the primary form coming along with it from the light and color that are in the body upon which the form shines. But this situation is quite rare, and for that reason such a form is rarely seen; moreover, among those [that are seen] only the form of intense, brilliant colors appears when the light that shines upon those colors is intense, and when those forms shine upon facing, white bodies, and when the light shining upon those bodies is weak in relation to those forms. Whatever is not of this sort does not appear.

6.105 ◉ Et similiter lux debilis que non apparet super corpus oppositum sibi est quia corpus oppositum luci debili, quando fuerit illuminatum ab alio lumine, admiscebuntur due luces, et sic non distinguetur lux debilis visui. Et cum corpus oppositum luci debili fuerit obscurum, non apparebit forma lucis debilis super ipsum, quoniam forma lucis est debilior ipsa luce, et forma secunda veniens ad oculum ab ista forma, ex qua oportet visum comprehendere istam formam super corpus oppositum luci, est debilior ista forma. Cum ergo lux fuerit debilis et corpus oppositum fuerit obscurum, erit forma que est super corpus oppositum valde debilis, et erit forma secunda que venit ex illa ad visum in fine debilitatis. Visus autem non comprehendit lucem que est in fine debilitatis.

6.105 ◉ Likewise, the failure of feeble light to appear upon a body facing it is due to the fact that, when the body facing the feeble light is lit by another [more intense] light-source, the two lights will mingle, and therefore the feeble light will not be [properly] discerned by the eye. But when the body facing the feeble light is dark, the form of the feeble light will not appear upon it because the form of the light is weaker than the light itself, and the secondary form reaching the eye from that form, by whose mediation the eye must perceive the form [shining] upon the body facing the light, is weaker than that form. Therefore, if the light is feeble and the facing body is dark, the form that shines upon the facing body will be very weak, and the secondary form that reaches from it to the eye will be weak to the point of vanishing. And the eye does not perceive light that is weak to the point of vanishing.

6.106 ◉ Forme ergo omnium colorum illuminatorum et forme omnis lucis oriuntur super corpora opposita, et non apparent plures illarum visui propter causas quas diximus. Et quedam apparent quando fuerint secundum modum quem narravimus. Iam ergo declarata est causa propter quam non comprehendit visus formas omnium colorum que sunt in corporibus coloratis super omnia corpora opposita illi, et comprehendit quasdam, et cum hoc comprehendit omnes colores qui sunt in corporibus coloratis. Et causa est quia comprehendit colores qui sunt in corporibus coloratis ex propria forma venienti ad ipsum ex eis que est fortior forma secunda venienti ad ipsum ex formis colorum qui sunt super corpora opposita illi. Et comprehendit formam colorum etiam singularem non admixtam cum alia, et comprehendit secundam formam provenientem ad ipsum ex formis colorum earum admixtam cum alia.

6.106 ◉ Hence, the forms of all illuminated colors and the forms of every light shine upon facing bodies, but several of them do not appear to the eye for the reasons we have enumerated. But some of them do appear when they conform to the conditions we have discussed. Therefore, the reason why the eye does not perceive the forms of all the colors in colored bodies [shining] on all bodies facing them but perceives some and thereby perceives all the colors in the colored bodies has now been demonstrated. And the reason is that it perceives the colors in colored bodies from the actual form reaching it from them, that form being stronger than the secondary form reaching it from the forms of the colors that are on the bodies facing them. And it also perceives the [primary] form of the colors separately, not mingled with others, whereas it perceives the secondary form that reaches it from the forms of their colors mingled with others.

6.107 ◉ Et hoc est quod promisimus declarare in fine capituli tertii, et declaratum est modo quod colores quos comprehendit visus ex rebus visis non comprehendit ipsos nisi admixtos cum formis lucis que sunt in eis et admixtos cum omnibus formis orientibus super ipsos ex coloribus corporum oppositorum. Et si in corpore diafono quod est medium inter ipsos et visum fuerit aliqua spissitudo, admiscebitur color eius etiam cum eis, et visus non comprehendit illum colorem singularium. Sed tamen forme que oriuntur super corpora colorata sunt in maiori parte valde debiles, et forme secunde que veniunt ex eis ad visum sunt in fine debilitatis. Et propter hoc erunt colores corporum ipsorum pluries vincentes super formas orientes super ipsa. Et similiter si in corpore diafono quod est medium inter visum et rem visam fuerit modica spissitudo, non distinguetur visui color eius a colore visi venientis cum eo quando color visi venientis cum eo fuerit fortior colore illius.

6.107 ◉ And this is what we promised to show at the end of the third chapter,⁑ and it has now been shown that sight only perceives the colors of visible objects mingled with the forms of light that are in them and mingled with all the forms shining upon them from the colors of facing bodies. Moreover, if there is some opacity in the transparent medium intervening between them and the eye, its color will also mix with those colors, and the eye does not perceive that color separately. Nonetheless, the forms that shine on colored bodies are, on the whole, very weak, and the secondary forms coming from them to the eye are weak to the point of vanishing. On account of this, the colors of the bodies themselves will generally overwhelm the forms [of color] shining upon them. Likewise, if there is a modicum of opacity in the transparent medium intervening between the eye and the visible object, its color will not be distinguished by the eye from the color of the visible object that comes with it when the color of the visible object that accompanies it is stronger than its color.

6.108 ◉ Quare vero lux fortis prohibet visum a comprehensione quarumdam rerum visarum est quia forme que veniunt ad visum super unam verticationem non comprehenduntur a visu nisi admixte. Et cum quedam forme admixte fuerint fortis scintillationis et quedam debilis, superabit forma fortis formam debilem, et sic non comprehendetur forma debilis a visu. Et cum forme admixte fuerint propinque in fortitudine, comprehendentur a visu, et erit comprehensio cuiuslibet illorum secundum illud quod admiscebitur cum eis ex formis admixtis cum eis, quoniam forme admixte non comprehenduntur a visu singulariter sed admixte.

6.108 ◉ But the reason intense light prevents the eye from perceiving certain visible objects is that the forms that reach the eye along one [radial] line are only perceived as mixed by the eye. And if some of the mixed forms dazzle while others are faintly radiant, the bright form will overwhelm the weak form, and the weak form will thus not be perceived by the eye. But when the forms that are mingled are of nearly the same strength, they will be perceived by the eye, but each of them will be perceived according to how the other forms that mingle with it will be mixed up with it, for mixed forms are perceived as mixed, not separately, by the eye.

6.109 ◉ Stelle ergo non comprehenduntur a visu in luce diei quia lux que pervenit in aere est fortior luce stellarum. Cum ergo inspiciens aspexerit celum in luce diei, erit aer qui est inter ipsum et celum illuminatus a lumine solis et continuatus cum visu, et erunt stelle ex posteriori illius lucis. Erunt ergo forma stelle et forma lucis que est in aere medio inter visum et illam stellam venientes ad visum super unam verticationem, et sic comprehendentur admixte. Sed forma lucis diei in aere est fortior multo forma lucis stelle, quare superabit lux aeris lucem stelle, et sic non distinguetur forma stelle.

6.109 ◉ Hence, the stars are not perceived by the eye during daylight because the light that pervades the air [at that time] is more intense than starlight. When a viewer looks up into the sky during daylight, then, the air between him and the heavens will be illuminated by sunlight and will be perfectly contiguous with the [surface of the] eye, and the stars will lie behind that light.⁑ Thus, the form of a star and the form of the light in the air intervening between the eye and that star will reach the eye along one [and the same radial] line, so they will be perceived as mixed. But the form of daylight in the air is considerably stronger than the form of the starlight, so that the light in the air will overwhelm the starlight, and thus the form of the star will not be [properly] discerned.

6.110 ◉ Et similiter lux debilis que est in medio fortis lucis— sicut ignis debilis in luce solis, et sicut noctiluca in luce diei, et sibi similibus; ista visibilia quando fuerint in luce solis aut diei, venient forme eorum ad visum admixte cum forma lucis fortis orientis super ipsas. Et comprehendet visus formam huiusmodi rerum visarum admixtam cum forma lucis fortis, quare superabit forma lucis fortis super formam debilem.

6.110 ◉ The same holds for a faint light that is in the midst of intense light—e.g., a faint fire in sunlight, or a firefly in daylight, or the like. When such visible objects are in sunlight or in daylight, their forms will come to the eye mixed with the form of the intense light shining upon them. And since the eye will perceive the form of such visible objects mixed with the form of the intense light, the form of the intense light will overwhelm the form of the faint light.

6.111 ◉ Et multotiens latet lux debilis et forma rei vise debilis quando pervenerit in visum lux fortis, quamvis non sit perventus duarum formarum ad visum ex una verticatione. Et hoc erit quando perventus duarum formarum fuerit ex duabus verticationibus vicinantibus et pervenerit in visum in duabus partibus vicinantibus. Et hoc apparet nocte in luce ignis, quoniam visus, quando comprehenderit lucem ignis, et fuerit ignis propinquus visui, et fuerit lux eius fortis, et fuerit in oppositione visui in illa dispositione aliquod visibile in quo est lux debilis accidentalis, et fuerit illud visibile remotius a visu igne, et fuerit super verticationem vicinantem verticationi ignis et propinquum verticationi ignis, tunc visus non comprehendet illud visibile comprehensione vera. Et si aspiciens cooperuerit ignem a suo visu aut removerit se a verticatione ignis ita quod sit verticatio a qua comprehendit illud visibile remota a verticatione ignis, tunc comprehendet illud visibile comprehensione manifestiore.

6.111 ◉ Moreover, a faint light or a weak form of a visible object is frequently unseen when intense light shines on the eye, even though the two forms do not reach the eye along the same [radial] line. This will be the case when the two forms radiate along neighboring [radial] lines and reach the eye at two neighboring spots [on its surface]. And this becomes clear at night in firelight, for, when the eye perceives the firelight and the firelight is near the eye so that its light is intense, and when there is some visible object facing the eye in that situation, and it is illuminated by faint, accidental light, and when that visible object is farther from the eye than the fire and lies along a line-of-sight near the fire’s line-of-sight, then the eye will not perceive that visible object properly. If, however, the viewer shields the fire from his sight or moves his line-of-sight with respect to the fire so that the line-of-sight along which he perceives that visible object lies far from the fire’s line-of-sight, then he will perceive that visible object more clearly [than before].⁑

6.112 ◉ Et causa illius est quod visibile in quo est lux debilis accidentalis habet formam obscuram, et cum ipsam comprehenderit visus et non comprehenderit cum ea lucem fortem, sentiet lucem debilem in quo est aliquid obscuritatis inter visum aut privationem lucis fortis a parte eius in quam pervenit forma lucis debilis. Et cum visus comprehenderit formam lucis debilis et comprehenderit cum ea lucem fortem, tunc etiam comprehenderit lucem fortem in parte ipsius contingenti partem qua comprehendebat formam obscuram. Non comprehendet visus lucem debilem que est in forma obscura propter duo: quorum unum est quod lux fortis, quando pervenerit in visum, illuminatur totus visus, et cum totus visus fuerit illuminatus, non apparebit in eo lux debilis, et maxime quando lux debilis fuerit proportionis minime respectu lucis fortis; et alterum est coniunctio lucis debilis cum luce forti in duabus partibus vicinantibus visus. Et lux debilis respectu lucis fortis est fere obscuritas, et cum lux vicinabitur ad formam obscuram debilem, et forma lucis fortis fuerit in visu, non comprehendet visus formam que est in luce obscura, nec comprehendet etiam formam obscuram nisi obscuritatem tantum; et sic non distinguetur ab eo forma, nec comprehendet eam comprehensione vera.

6.112 ◉ The reason for this is that the visible object possessing the faint accidental light has a dark form, and when the eye perceives that form without perceiving intense light along with it, it will sense the faint light, given that there is some darkness between the eye [and the object] or an absence of intense light on the side of it where the form of the weak light reaches. But when the eye perceives the form of faint light while it perceives the form of intense light along with it, then it perceives the intense light at a spot on the eye that is next to the spot at which it perceived the dark form. The eye will [therefore] not perceive the faint light in the dark form for two reasons: first, because when intense light reaches the eye, the entire eye is illuminated, and when the entire eye is illuminated, faint light will not appear in it, particularly when the [intensity of the] faint light is minimal in comparison to [that of the] intense light;⁑ and second, [because] the faint light abuts on the intense light at the two neighboring spots on the eye. But the faint light is almost dark in comparison with the intense light, so when the [intense] light lies next to the weak, dark form while the form of the intense light floods the eye, then the eye will not perceive the form that is faintly illuminated, and all it will perceive of a dark form is its darkness; and so the form will not be [properly] discerned by the eye, nor will the eye perceive it properly.

6.113 ◉ Et occultatio formarum debilis lucis propter vicinitatem lucis fortis habet simile in coloribus, quoniam color fuscus, quando intinguetur cum eo corpus album punctatim, puncta apparebunt nigra propter fortitudinem albedinis. Et si eadem puncta fuerint posita super corpora valde nigra, apparent fere alba, et non apparebit obscuritas que est in eis. Et quando illa tinctura fuerit in corporibus que non sunt multum alba nec multum nigra, apparebit color secundum suum esse.

6.113 ◉ Moreover, the overshadowing of the forms of faint light because of the nearness of intense light has its counterpart in colors, for when spots of some relatively dark color are painted on a white body, the spots will appear black because of the intensity of the [body’s] whiteness. But if identical spots are painted on a pitch-black body, they will appear almost white, and their darkness will go unseen. But when a color is painted on bodies that are neither intensely white nor pitch-black, the color will be seen as it really is.

6.114 ◉ Et similiter quando color viridis segetalis fuerit super corpus citrinum, apparebit illa tinctura obscura; et quando fuerit in corpore nigro, apparebit illa tinctura similis colori origani, et similiter omnis tinctura media inter duas extremitates.

6.114 ◉ By the same token, when a grass-green color is painted on a yellow body, it will appear dark, but when it is painted on a black body, it will appear the color of wild marjoram, and the same holds for all colors that lie midway between two extremes.⁑

6.115 ◉ Visibilia ergo vicinantia quando fuerint remota in fortitudine et debilitate coloris, quod est debilis coloris latebit visum, quoniam qualitates lucis et coloris non comprehendentur a visu nisi ex respectu eorum adinvicem. Et lux fortis non prohibebit visum a comprehensione visibilium lucis debilis nisi propter admixtionem forme lucis debilis cum formis earum, et propter victoriam formarum lucis fortis super formas lucis debilis, et debilitatem sensus ad comprehendendum illud quod est minime proportionis respectu fortis.

6.115 ◉ Thus, when neighboring visible objects differ sharply in the intensity or faintness of their color, the faint color will be unseen by the eye, because the qualities of light and color will be perceived only with respect to others around them. And intense light will prevent the eye from perceiving faintly illuminated visible objects only because of the mingling of the form of the weak light with its form, as well as because of the predominance of the forms of intense light over the forms of faint light and the inability of the sense to perceive anything whose intensity is minimal in comparison to that of something else.

6.116 ◉ Iam ergo complevimus declarationem omnium rerum dependentium ab isto capitulo.

6.116 ◉ Accordingly, we have now accounted for all the subjects that bear on this chapter.

◉[Capitulum 8]

◉[Chapter 8]

7.1 ◉ Tunice quas diximus in declaratione forme visus sunt instrumenta per que completur visio.

7.1 ◉ The tunics that we discussed in our account of the structure of the eye serve as instruments through which vision is achieved.

7.2 ◉ Tunica vero prima, que dicitur cornea, est tunica diafona et cum hoc fortis, et est superposita foramini quod est in anteriori uvee. Et prima utilitas eius est quia cooperit foramen uvee, quare retinet humorem albugineum quod est in anteriori uvee. Et est diafona ut transeant in ea forme lucis et coloris ad interius visus, quoniam non transeunt nisi per diafona. Fortitudo autem eius est ut non corrumpatur cito, quoniam est exposita aeri et potest cito corrumpi ex fumo, et pulvere, et sibi similibus.

7.2 ◉ Now the first tunic, which is called the cornea, is a transparent but tough membrane, and it extends over the opening in the anterior of the uvea. Its primary function is to cover the opening in the uvea so as to keep the albugineous humor, which lies in front of the uvea, in place. It is transparent so that the forms of light and color can pass through it into the interior of the eye, for they only pass through transparent bodies. Its toughness, moreover, is meant to keep it from deteriorating easily, for it is exposed to air and can easily be damaged by smoke, dust, and the like.

7.3 ◉ Humor autem albugineus est diafonus, et est hic humidus fluxibilis. Diafonitas autem est ut pertranseant in eo forme et perveniant in eo ad humorem glacialem. Humiditas autem eius est ut semper humefaciat humorem glacialem, ita ut eius natura sit custodita, quoniam tela que est super glacialem est valde tenuis, et minima siccitate poterit corrumpi.

7.3 ◉ The albugineous humor, for its part, is transparent, and it is quite fluid. It is transparent in order to let forms pass into it and extend through it to the glacial humor. It is moist, however, in order to keep the glacial humor continually moist so that it can maintain its proper condition, for the membrane that covers the glacialis is extremely thin, and the least dryness could damage it.

7.4 ◉ Tunica autem nigra continens humorem albugineum, que est uvea, est nigra, et fortis, et spissa, et sperica, et in anteriori eius est foramen rotundum, sicut narravimus. Nigredo vero eius est ut obscuretur humor albugineus et glacialis ita quod appareant in ea forme lucis debilis, quoniam lux debilis valde apparet in locis obscuris et latet in locis luminosis. Et est aliquantulum fortis ut retineat humorem albugineum et ut non resudet ex eo aliquid ad exterius. Et est spissa ut sit obscura, quoniam si esset rara, esset diafona; et cum erit spissa, obscurabitur anterior pars eius. Et est sperica quia est magis temperata figurarum, et est magis remota ab occasionibus, et habens angulos citius alteratur per angulos. Foramen autem quod est in anteriori istius tunice est ut pertranseant ipsam forme ad interius visus, et est rotundum quia rotunditas est simplicior figurarum et amplior ysoperimetrorum.

7.4 ◉ Now the black tunic, i.e., the uvea, that contains the albugineous humor, is black, tough, thick, and spherical, and in its front there is a round opening, as we pointed out earlier.⁑ It is black in order to darken the albugineous and glacial humors so that the forms of faint light can appear in them, for faint light definitely appears in dark locations but is invisible in brightly illuminated locations. Moreover, it is somewhat tough in order to hold the albugineous humor in place so that none of it leaks out. It is thick in order to be opaque, for if it were thin, it would be translucent; but since it is thick, its inner side will be dark.⁑ It is spherical because the sphere is the most efficient of shapes and is least susceptible to injury, whereas figures that have corners are easily altered at [those] corners. There is an opening at the front of this tunic so that forms can pass through it into the interior of the eye, and this opening is circular because the circle is the simplest and most capacious figure of all figures having the same circumference.⁑

7.5 ◉ Humor autem glacialis habet multas proprietates per quas completur sensus. Quoniam est humidus et subtilis, et est in eo aliquid diafonitatis et spissitudinis. Et super ipsum est tela valde rara, et figura superficiei eius est composita ex duabus superficiebus spericis diversis, et anterior illarum est maioris spericitatis altera. Est autem humidus ut citius patiatur a luce, et est subtilis quia talia corpora sunt subtilis sensus. Et est aliquantulum diafonum ut recipiat formas lucis et coloris et ut pertranseant per ipsum lux et color, et est aliquantulum spissus ut remaneant in eo diu forme lucis et coloris ita quod appareat virtuti sensibili forma lucis et coloris que figebantur in eo. Et si esset diafonum in fine diafonitatis, pertransirent forme in eo, et non pateretur a formis passione que est ex genere doloris, et sic non comprehenderet formas.

7.5 ◉ The glacial humor has many characteristics that help bring [visual] sensation about. For it is moist and subtle, and it possesses some transparency as well as some opacity. Covering it is a very thin membrane, and its surface takes shape as a composite of two different spherical surfaces, the anterior of which is more gradually curved than the posterior.⁑ It is moist so that it can more easily suffer the effect of light, and it is subtle because such bodies are exquisitely sensitive. It is, moreover, somewhat transparent so that it can receive the forms of light and color and so that light and color can pass through it, but it is somewhat opaque so that the forms of light and color can persist in it for awhile in order to let the form of the light and color impressed in it be seen by the sensitive faculty.⁑ If it were perfectly transparent, though, the forms would pass through it, but it would not feel the effect of the forms, which is of the nature of pain, and so it would not perceive those forms.

7.6 ◉ Tela autem que est super istum humorem est ut retineat ipsum quatinus non fluat, quoniam humores nisi retinerentur aliquo, fluerent et non remanerent secundum unam figuram. Et ista tela est valde rara ut non occultet formas venientes. Et est sperica propter causam quam diximus, et superficies anterioris eius est ex spera maiori ut sit equidistans superficiei anteriori visus, ita quod centra illarum sit unus punctus.

7.6 ◉ The membrane that covers this humor is there to constrain it so that it does not flow, for unless something constrained them, the humors would flow and would not maintain a constant shape. But this membrane is exceedingly rare so that it will not block out the incoming forms. It is spherical for the same reason we mentioned earlier, and its anterior surface is formed from a great sphere so as to be parallel to the anterior surface of the eye in order that the centers of both [surfaces] form a single point.

7.7 ◉ Nervus autem obticus super quem componitur oculus totus est obticus ut currat per ipsum spiritus visibilis a cerebro, et perveniat ad glacialem, et det ipsi virtutem sensibilem successive, et ut pertranseant etiam forme in corpore subtili currenti in suo concavo quousque perveniant ad ultimum sentiens quod est in anteriori cerebri.

7.7 ◉ The hollow nerve to which the whole eye is attached is hollow so that the visual spirit can flow through it from the brain to reach the glacialis and thereby endow it in turn with sensitive power, and so that the forms can also pass through the subtle substance flowing through its hollow until they reach the final sensor at the front of the brain.

7.8 ◉ Et principia duorum nervorum super quos componuntur oculi duo sunt in duabus partibus anterioris cerebri ut situs duorum oculorum a suis duobus principiis sit situs consimilis. Et non fuit principium eorum a medio anterioris cerebri, quia iste locus est proprius sensui ordinatus.

7.8 ◉ And the wellsprings of the two nerves to which the two eyes are attached lie on both sides of the anterior part of the brain so that the location of the two eyes will correspond with the location of their two wellsprings. Their wellspring was not in the middle of the anterior part of the brain because this location is more appropriately designated for the sense [of smell].⁑

7.9 ◉ Quare enim fuerint duo oculi est benignitas operatoris ita quod, si uni illorum accideret occasio, remaneat alter, et ut forma faciei sit etiam pulcrior.

7.9 ◉ Indeed, there are two eyes because of the beneficence of the Creator⁑ [who chose to double the eyes] so that, if one of them were to be injured, the other would remain [functional], and also so that the face would look more comely [than it would with only one eye].

7.10 ◉ Causa autem propter quam concurrunt isti duo nervi iam fuit dicta in qualitate visionis.

7.10 ◉ The reason, moreover, that the two [optic] nerves join has already been given in [the section on] how vision occurs.⁑

7.11 ◉ Superficies vero tunicarum oculi sunt sperice et equidistantes, et centrum illarum est unum punctum ita quod perpendicularis que est super primam illarum est perpendicularis etiam super omnes. Et sunt sperice ut exeant omnes perpendiculares ab uno puncto quod est centrum illarum, deinde distent apud extremitates secundum remotionem a centro ita quod piramis extensa a centro contineat omnes perpendiculares exeuntes ab illa re visa et distinguat ex superficie visus et membri sentientis partem parvam continentem, quamvis sit parva, totam formam venientem a re visa ad visum. Et si superficies tunicarum visus essent plane, non veniret forma visi ad visum super perpendiculares nisi esset visus equalis viso. Et nulla figura est in qua adunantur perpendiculares et concurrunt in unum punctum eius, et est superficies super quam elevantur equalis ordinationis, nisi figura sperica.

7.11 ◉ The surfaces of the tunics of the eye are spherical and parallel, and their centers coincide at one point so that [any line] perpendicular to the first of them is perpendicular to all. And they are spherical so that [all their] perpendiculars may issue from the single point that forms their center and then diverge as they part from the center in order that the cone projected from that center can contain all the perpendiculars extending from any visible object [to that centerpoint] and can demarcate a small spot on the surface of the eye as well as on the sensitive organ, that spot, no matter its smallness, being able to encompass the entire form reaching from the visible object to the eye. If, however, the surfaces of the tunics of the eye were flat, the form of the visible object would not reach the eye along perpendiculars unless the eye were the same size as the object. But there is no other figure than the sphere in which the perpendiculars come together and meet at a single point and upon whose surface those perpendiculars fall in perfect order.

7.12 ◉ Et cum ista dispositione possunt exire a centro visus multe piramides ad multa visa in eodem tempore, et quelibet illarum distinguet partem parvam superficiei membri sentientis continentem formam illius visi. Et omnes tunice habent unum centrum propter illud quod prediximus, et est ut perpendiculares exeuntes a re visa ad unam illarum sint perpendiculares super omnes et ut pertranseant etiam forme omnes secundum unam verticationem.

7.12 ◉ According to this disposition many cones can extend at the same time to many visible objects from the center of the eye, and each of them will demarcate a small section on the surface of the sensitive organ that encompasses the [whole] form of that visible object. And all the tunics have a single center for the reason we have given before, that reason being so that the perpendiculars issuing from the visible object to one of those tunics will be perpendicular to all of them and so that forms may pass through all of them along a single [radial] line.

7.13 ◉ Quare vero nichil comprehendit visus ex rebus visibilibus nisi ex verticationibus istarum perpendicularium tantum est quia per istas perpendiculares tantum ordinantur partes vise rei in superficie membri sentientis. Et hoc fuit iam manifestum ante quoniam non potest ordinari forma rei vise in superficie membri sentientis nisi sit receptio eius ad formam ex istis verticationibus tantum. Et propter hoc appropriatur natura visus ista proprietate, et naturatur quod non recipiat aliquam formam nisi secundum situm istarum verticationum tantum. Et appropriatio visus habita hac proprietate est una rerum ex quibus apparet maxima discretio operatoris et bonitas preparationis nature in preparando instrumenta visus et formam per quam completur sensus et per quam distinguuntur visibilia.

7.13 ◉ Now the reason sight perceives visible objects only along such perpendiculars is that it is only according to such perpendiculars that the parts of the visible object are properly arranged on the surface of the sensitive organ. And it was already shown earlier that the form of a visible object cannot be properly arranged on the surface of the sensitive organ unless the form is received along these [radial] lines alone.⁑ Accordingly, this is an intrinsic characteristic of the eye, so it is naturally constituted not to receive any form except along these [radial] lines. And the fact that the eye is endowed with this property is one of the things manifesting the incredible perspicacity of the Creator and the providence of nature in designing the instruments of sight and the arrangement according which [visual] sensation is achieved and visible objects are discerned.

7.14 ◉ Consolidativa autem continet omnes istas tunicas; et in ea est aliquid humiditatis, et cum hoc habet aliquid retentionis, et est aliquantulum fortis. Et continet istas tunicas ut congreget et conservet illas, et est aliquantulum humida ut preparentur loca tunicarum ex ea et ut non accidat siccitas velociter illis tunicis. Et est aliquantulum retentiva et fortis ut conservet situs et figuras tunicarum ut non alterentur cito. Et est alba ut sit per ipsam forma faciei pulchra.

7.14 ◉ The sclera encloses all of these tunics; and there is some moisture in it, yet it also has some firmness, and is somewhat tough. It encloses all of these tunics in order to keep them together and to preserve them, and it is somewhat moist so that the locations of the tunics can thereby be prepared and so that those tunics cannot be quickly dried out. It is somewhat firm and tough so that it can keep the tunics in place and have them maintain their [spherical] shape so that they will not be readily subject to change. It is white so that the face will be comely on that account.⁑

7.15 ◉ Et totus oculus est rotundus quoniam rotunditas est melior figurarum et maior et levioris motus. Oculus autem indiget motu et velocitate motus ita quod sit oppositus per motum multis visibilibus in eodem tempore, et ut sit oppositus propter motum omnibus partibus rei vise medium aspicientis ita quod comprehendat ipsum comprehensione vera et cum hoc consimili, quoniam sensus per medium membri sentientis est manifestior (et hoc declarabimus post in loco convenienti). Velocitas autem motus visus est ut sit aspiciens omnes partes rei vise et visibilia sibi opposita in modico tempore.

7.15 ◉ The entire eyeball is round because roundness represents the best, most capacious, and most easily moved of shapes. The eye, however, needs to move, and to move quickly, so that by moving it can face many visible objects at the same time and so that, by moving, the viewer’s central [line-of-sight] can face all parts of a visible object in order to perceive it with a true and consistent perception, for sensation through the middle of the sensitive organ is most clear (we will demonstrate this later in a suitable place).⁑ The eye moves quickly so that in very short order it can see all the parts of a visible object as well as [all] the visible objects facing it.

7.16 ◉ Palpebre autem sunt ut conservent oculum apud sompnum et ut faciant oculum quiescere quando fatigatur a lumine, quoniam luces fortes nocent oculis, et si continue aperirentur oculi supra modum, debilitarentur. Et hoc apparet quando oculi aspiciunt lucem fortem longo tempore. Et similiter nocet visui aer quando in eo fuerit fumus aut pulvis. Palpebre ergo cooperiunt oculum a luce, quando indigent hoc, et conservant ipsos ab aere, et abstergunt ab eis multa nocumenta. Deinde quando fatigantur, superponuntur palpebre ita quod completur in eis sua requies, et sunt velocis motus ut citius superponantur oculis apud appropinquationem nocumentorum oculis.

7.16 ◉ The eyelids are designed to preserve the eye during sleep and to keep the eye still when it is fatigued by light, for intense light harms the eyes, and if the eyes are continually open to it, they will be debilitated. This is obvious when the eyes stare at an intense light for a long time. Likewise, when there is smoke or dust in it, air harms the eye. Thus, the eyelids shield the eye from light when the eyes need it, and it protects them from the air and wipes away many harmful residues from them. Then, when the eyes are tired, the eyelids are closed over them so that they can finish resting, and the eyelids move quickly so that they can close over the eyes as soon as anything harmful approaches the eyes.

7.17 ◉ Cilia autem sunt ad temperandam quandam partem lucis quando dolebit visus propter fortitudinem lucis, et propter hoc adunat aspiciens oculum suum et constringit ita quod possit aspicere ab angusto quando lux fortis nocuerit ei.

7.17 ◉ The eyelashes are there, however, to mitigate some of the light when it will hurt the eye because of its intensity, and for this reason the viewer squints his eye and narrows it so that he can see from a narrow field of vision when intense light would hurt it.

7.18 ◉ Ista ergo que diximus sunt utilitates instrumentorum visus ex quibus manifestatur magna discretio operatoris. Sit ergo nomen eius benedictum et bonitas preparationis nature.

7.18 ◉ These things we have discussed cover the functions of the instruments of vision, from which the great perspicacity of the Creator is manifest. Let his name therefore be blessed, along with the goodness of nature in its providential order.

◉[Capitulum 9]

◉[Chapter 9]

8.1 ◉ Iam ergo declaratum est superius quod visus nichil comprehendit ex rebus visis que sunt cum eo in eodem aere (ita quod comprehensio eorum ab eo non sit secundum reflexionem) nisi quando aggregabuntur iste res, et sunt: [1] ut sit inter ea aliquid spatii, [2] et ut sit opposita visui illa resscilicet ut sit inter quodlibet punctum eius superficiei quam comprehendit visus et inter aliquod punctum superficiei visus linea recta ymaginabilis, [3] et ut sit in ea lux, [4] et ut sit corpus eius aliquantulum in respectu virtutis sensus visus, [5] et ut sit aer medius diafonus continue diafonitatis, et ut non sit in eo aliquod corpus non diafonum, [6] et ut sit res visa resistens visuiscilicet ut non sit in ea diafonitas, aut si sit, sit spissior diafonitate aeris medii inter ipsam et visum; sed tale non potest esse nisi cum colore aut eius simili. Visus autem non comprehendet rem visam nisi quando congregabuntur iste sex intentiones, et si res visa caruerit istarum una intentionum, non comprehendetur a visu.

8.1 ◉ It has been demonstrated earlier⁑ that the eye perceives none of the visible objects that occupy the same air with it (provided that it does not perceive them by means of broken rays) unless the following conditions are met, namely: (1) that there be some space between eye and object, (2) that the object face the eye—i.e., that a straight line can be imagined extended between any point on the surface of the visible object perceived by the eye and some point on the surface of the eye, (3) that the object possess some illumination, (4) that it have some [perceptible] size with respect to the eye’s sense-capacity, (5) that the aereal medium be continuous and transparent and that there not be any opaque body in it [between eye and object], and (6) that the visible object block sight—i.e., that there be no transparency in it, or if there is, that it be more opaque than the air intervening between it and the eye; but this can only happen with color or the like. Furthermore, sight will not perceive a visible object unless these six conditions are met as a whole; if the visible object fails to meet any one of these conditions, it will not be perceived by sight.

8.2 ◉ Indigentia autem visus ab unaquaque istarum intentionum non est nisi propter aliquam causam.

8.2 ◉ Each one of these conditions is necessary to sight for some specific reason.

8.3 ◉ Quare ergo non comprehendit visus rem visam nisi quando inter ea fuerit distantia aliqua, et non comprehendit ipsam quando applicabitur ei est propter duas causas, quarum una est quod visus non comprehendit rem visam nisi quando fuerit in ea aliqua lux. Et quando fuerit applicata visui et non fuerit illuminata per se, non erit in sua superficie vicinanti visui lux, quoniam corpus oculi secundum situm suum tunc prohibetur a visu. Res autem luminose per se non possunt applicari superficiei visus, quoniam res illuminate per se sunt stelle et ignis que non possunt applicari superficiei visus. Causa autem secunda est quod visio non erit nisi ex parte opposita foramini uvee ex medio superficiei visus, et cum res visa applicabitur visui, non superponetur isti parti visui nisi pars equalis illi tantum ex re visa. Et si visus comprehendet rem visam per applicationem, non comprehendet nisi partem applicatam parti opposite foramini tantum, et non comprehendet residuum rei vise. Et si moveatur res visa super superficiem visus quousque contingat totam superficiem rei vise secundum partem mediam visus, comprehendet partem post aliam, et dum comprehendet partem secundam non comprehendet partem primam, et sic non poterit comprehendere totam rem in simul. Et cum ita est, non figurabitur in eo forma rei vise ita quod, si aliqua res visa esset super corpus densum et esset in illo corpore denso foramen minoris quantitatis re vise, et res visa esset applicata foramini, non comprehenderet ex ea nisi partem superpositam foramini tantum. Deinde si res visa moveatur super foramen quousque comprehendatur a visu pars post aliam, non figuratur in visu tota forma eius.

8.3 ◉ Accordingly, the reason that the eye perceives a visible object only when there is some separation between eye and object but does not perceive it when it is placed directly upon it eye is twofold. First, the eye does not perceive a visible object unless there is some light in it. But if that object is placed directly upon the eye and has no intrinsic luminosity, there will be no light on its surface where it touches the eye, for, by its position, the body of the eye will be prevented from seeing it.⁑ On the other hand, an object that is intrinsically luminous cannot be placed upon the surface of the eye because intrinsically luminous bodies include the stars and fire, which cannot be placed upon the eye.⁑ The second reason is that vision will only occur on the side facing the opening in the uvea at the center of the eye’s surface, but when a visible object is placed on the eye, the area of the object that touches the eye will only be the size of the area it touches on the eye. But if the eye perceives the visible object through direct contact, it will perceive only that part directly touching the opening but will not perceive the rest of the visible object. And if the visible object is passed over the surface of the eye until the eye touches the entire surface of the visible object at the center of its own surface, it will perceive the object one part at a time, and when it perceives the second part it will not perceive the first part, so it will be unable to perceive the whole object at once. Further, if that is the case, the form of the [entire] visible object will not be delineated in it [but will appear] much as [would be the case] if some visible object were placed on an opaque body, and there were an aperture smaller than the visible object in that opaque body, and the visible object were placed at the opening; [for] in that case only the part of the object placed at the aperture would be perceived. Then, if the visible object were moved over the aperture until it was perceived bit-by-bit by the eye, its whole form would not be delineated in the eye.

8.4 ◉ Si ergo visio esset per contactum, non comprehenderet visus totam rem visam nec figuram et formam eius nisi esset res visa equalis parti medie superficiei visus per quam erit visio, et cum hoc non potest comprehendere res visas multas in eodem tempore. Et cum inter visum et rem visam fuerit aliquod spatium, poterit comprehendere totam rem visam in eodem tempore ex aliqua parte parva, quamvis res visa sit magna; et potest comprehendere res visas multas simul in eodem tempore. Et cum res visa fuerit remota a visu erit possibile lucem oriri super superficiem eius oppositam visui. Propter ergo istas duas causas non comprehendit visus ex rebus visibilibus nisi sit inter eos aliquod spatium.

8.4 ◉ Hence, if vision were [to take place] through physical contact, the eye would not perceive the entire visible object nor [would it perceive] its shape and arrangement unless the visible object were the same size as the central spot on the eye through which vision would occur; and, in addition, it could not perceive several visible objects at the same time. But when there is some space between the eye and the visible object, the eye can at once perceive the entire visible object at a small spot [on its surface], even if the visible object is large; and it can perceive several visible objects at the same time. Furthermore, when the visible object is separated from the eye, it will be possible for light to shine on the surface of that object facing the eye.⁑ For these two reasons, then, sight does not perceive visible objects unless there is some space between them and the eye.

8.5 ◉ Quare vero non comprehendit visus rem visam que est cum eo in eodem aere et in parte opposita illi nisi sit inter quodlibet punctum eius et aliquod punctum superficiei partis per quam erit visio ex superficie visus linea recta est quia declaratum est quod visio non erit nisi ex formis venientibus a re visa ad visum, et quod forme non comprehenduntur nisi secundum lineas rectas. Et propter hanc causam non comprehendit visus rem nisi sit inter ea linea recta. Et si secuerint densa media corpora omnes lineas que sunt inter ea, latebunt res vise visum, et si secuerit illud corpus quasdam illarum linearum rectarum, latebit visum quedam pars que est apud extremitatem linearum resectarum per corpus densum.

8.5 ◉ That sight perceives a visible object occupying the same air as the eye while facing it only if a straight line can be [imagined extended] between any point on the object and some point on the area of the eye’s surface where vision occurs is due to the following. It has already been shown that vision will not occur except through forms reaching the eye from the visible object and that forms are perceived only along straight lines.⁑ As a result, the eye does not perceive an object unless there is a straight line between it and the object. And if opaque bodies are interposed to cut all the [straight] lines between them, objects will disappear from sight, whereas if an opaque body interrupts [only] some of those straight lines, a certain part of the visible object at the endpoints of the [straight] lines interrupted by the opaque object will disappear from sight.

8.6 ◉ Quare vero visus non comprehendit rem visam nisi sit in ea lux est propter duas causas: aut quia forme coloris que sunt in rebus visis non extenduntur in aere nisi sit lux cum colore, aut quia forma coloris extenditur in aere, quamvis non sit cum ea lux, sed non operatur in visum operatione sensibili nisi per lucem. Et manifestum est quod forma lucis est fortior forma coloris, et quod lux operatur operatione manifestiori, et quod forma coloris, quia est debilis, non potest operari in visum sicut operatur lux. Et forma coloris que est in corpore illuminato semper est admixta cum forma lucis, et cum pervenerit ad visum, operatur in ipsum per suam fortitudinem et preparationem visus ut patiatur ex ea. Et quia admiscetur cum forma coloris et non distinguitur ab ea, non sentit visus formam lucis nisi admixtam cum forma coloris. Visus ergo non sentit colorem rei vise nisi ex colore admixto cum forma lucis veniente ad ipsum ex re visa, et propter hoc alterantur colores multarum rerum visarum apud visum per alterationem lucis orientis super ipsas. Quia ergo forma coloris non operatur in visum nisi sit admixta cum lumine, et non sit ex colore forma nisi sit in ea lux, nichil comprehendit visus ex rebus visibilibus nisi quando in ea fuerit aliqua lux.

8.6 ◉ Sight does not perceive a visible object unless it is illuminated for two reasons: either because the forms of the color in the visible objects do not radiate through the air except when light accompanies the color, or because the form of the color does radiate through the air, even though no light accompanies it, but does not make a perceptible effect upon the eye except by means of [accompanying] light. Now it is clear that the form of light is stronger than the form of color, that light has a more noticeable effect than color, and that, because it is weak, the form of color cannot affect sight the way that light does. But the form of color in an illuminated body is invariably mingled with the form of light, and, when it reaches the eye, it affects sight by virtue of its intensity as well as by virtue of the disposition of the eye to suffer its effect. But since light is mingled with the form of color and is not discerned separately from it, the eye only senses the form of light mingled with the form of color. Therefore, the eye senses the color of the visible object only on the basis of that color mingled with the form of the light reaching it from the visible object, and consequently, as far as sight is concerned, the colors of many visible objects vary according to variations in the light shining upon them.⁑ Therefore, since the form of color does not affect sight unless it is mingled with light, and since color does not generate a form unless it is illuminated, sight perceives no visible object unless it possesses some illumination.

8.7 ◉ Quare vero non comprehendit visus rem visam nisi sit corpus eius aliqua quantitate est quia declaratum est quod forma rei vise non pervenit ad visum nisi ex piramidibus cuius caput est centrum visus et basis superficies rei vise et quod ista piramis distinguit ex superficie membri sentientis parvam partem in qua ordinabitur forma rei vise. Et si res visa fuerit valde parva, erit piramis que est inter ipsam et centrum visus valde parva. Erit igitur pars distincta ex membro sentiente quasi punctum valde parva. Sed sentiens non sentit formam nisi quando pars sue superficiei ad quam pervenit forma fuerit quantitatis sensibilis respectu totius. Et virtutes sensus etiam sunt finite, et cum pars membri sentientis ad quam pervenit forma non est quantitatis sensibilis apud totum membrum sentiens, non sentiet passionem que accidit in illa parte propter parvitatem ipsius, quare non comprehendit formam. Res ergo visa que est possibilis comprehendi a visu est illa in qua erit piramis que figuratur inter visum et centrum visus distinguens ex superficie glacialis partem quantitatis sensibilis respectu totius superficiei glacialis. Et iste sensus erit secundum tantum ad quantum pervenit virtus sensitiva, et non extenditur ad infinitum, et diversatur etiam secundum diversitatem virtutis oculi. Et cum piramis que figuratur inter rem visam et centrum visus est distinguens ex superficie glacialis partem quantitatis insensibilis respectu totius superficiei glacialis, non potest visus comprehendere illam rem. Et propter hoc non comprehendet rem visam visus valde parvam.

8.7 ◉ Why sight does not perceive a visible object unless it has some [appreciable] size is explained in the following way. It has been shown that the form of a visible object reaches the eye by means only of cones whose vertex lies at the center of the eye and whose base is formed by the surface of the visible object and that such a cone demarcates a small area on the surface of the sensitive organ where the form of the visible object will be arranged.⁑ If the visible object is extremely small, the cone formed between it and the center of the eye will be extremely small. Accordingly, the area demarcated upon the sensitive organ will be so small as to be virtually a point. But the sensitive [organ] does not sense a form unless the area on its surface to which the form comes has a perceptible size in proportion to the whole [of the surface]. Moreover, sensitive powers are finite, so when the area of the sensitive organ to which the form comes does not have a perceptible size in proportion to the whole sensitive organ, it will not feel the effect made there because of its smallness, the result being that it does not perceive the form.⁑ Therefore, a visible object can be perceived by sight if the cone that is formed between the object and the center of the eye will demarcate an area on the surface of the glacialis that has a perceptible size in proportion to the whole surface of the glacialis. But the resulting sensation will depend entirely upon the extent of [the eye’s] sensitive power, which does not go on to infinity, and that power varies with the capacity of the [given] eye. But if the cone that is formed between the visible object and the center of the eye demarcates an area on the surface of the glacialis that has an imperceptible size in proportion to the entire surface of the glacialis, sight cannot perceive that object. It is for this reason that sight will not perceive an extremely small object.

8.8 ◉ Quare vero non comprehendit visus rem visam nisi quando corpus medians inter ipsam et visum fuerit diafonum est quia visio non est nisi ex forma venienti ex re visa ad visum. Forme autem non extenduntur nisi in corporibus diafonis, et visio non completur quando res visa fuerit cum visu in eodem aere (et fuerit comprehensio non secundum reflexionem) nisi quando aer fuerit continuus inter rem visam et visum, et non absciderit lineas rectas verticales que sunt inter ea corpus densum, quoniam forma non extenditur in aere consimilis diafonitatis nisi secundum lineas rectas. Et propter hoc non comprehendit visus rem visam que est cum eo in eodem aere et in parte opposita visui nisi quando aer medius inter eas fuerit diafonus consimilis diafonitatis.

8.8 ◉ That the eye does not perceive a visible object unless the medium intervening between that object and the eye is transparent is because vision only occurs by means of a form reaching from the visible object to the eye. But forms only extend through transparent bodies, so vision is achieved when the visible object occupies the same air as the eye (provided that the perception does not take place through broken rays) only if the air between the visible object and the eye is continuous and an opaque body does not interrupt the straight lines extending between them, for a form extends through air of uniform transparency only along straight lines. For this reason the eye perceives a visible object that occupies the same air with it and faces it only when the air between eye and object is of uniform transparency.

8.9 ◉ Quare vero visus non comprehendit rem visam nisi quando fuerit in ea densitas aut aliquid densitatis est propter duas causas, quarum altera est quia quod est densum est coloratum, et ex colore venit forma ad visum ex qua comprehendit visus colorem rei vise. Quod autem est in fine diafonitatis caret colore, quare non comprehenditur a visu. Et causa secunda est quoniam visus non comprehendit rem visam nisi sit illuminata, et veniat ex luce que est in ea forma secunda ad visum cum forma coloris. Et non erit forma secunda ex luce orienti super aliquod corpus nisi figatur lux in illo corpore super quod oritur. Cum ergo lux fuerit fixa in illo corpore, erit ex eo forma secunda; et quando lux orietur super corpus diafonum valde, non figetur in eo, sed extendetur in sua diafonitate. Cum ergo corpus diafonum fuerit oppositum visui et super ipsum oritur lux ex parte in qua est visus, in eo extendetur et non figetur in sua superficie. Et sic non erit in superficie opposita visui illius corporis lux ex qua venit forma ad visum. Et si illud illuminatum cuius lux oritur super illud corpus diafonum fuerit oppositum visui, pertransibit lux eius in corpus diafonum et perveniet ad visum, et nichil deferet cum eo ad visum ex colore corporis diafoni, quoniam corpus diafonum quod est in fine diafonitatis non habet colorem. Visus ergo comprehendet ex illo loco corpus illuminatum cuius lux oritur super corpus diafonum post corpus diafonum, et non comprehendet corpus diafonum propter hoc quia non comprehendit visus rem visam que est in fine diafonitatis. Et cum diafonitas corporis fuerit similis diafonitati aeris, erit eius dispositio sicut dispositio aeris, et non comprehendetur a visu, sicut nec aer et corpora diafona quorum diafonitas non est spissior diafonitate aeris non comprehendentur a visu, quoniam nulla forma venit ex eis ad visum que potest operari in visum. Et similiter erit si inter visum et rem visam fuerit medium corpus diafonum preter aerem et fuerit diafonitas rei vise non spissior diafonitate corporis medii.

8.9 ◉ There are two reasons why sight does not perceive a visible object unless it is [completely] opaque or possesses some opacity. One reason is that whatever is opaque is colored, and [it is] from color [that] the form by means of which sight perceives the color of a visible object comes to the eye. Whatever is absolutely transparent, however, lacks color, so it is not perceived by the eye. The second reason is that sight does not perceive a visible object unless it is illuminated and a secondary form of the light in it reaches the eye along with the form of its color. But there will be no secondary form of light shining on any object unless it is fixed in the object upon which it shines. Therefore, if the light is fixed in that body, a secondary form will radiate from it; but when light shines upon an exquisitely transparent body, it will not be fixed in it but will pass through its transparency. When a transparent body faces the eye, then, and when light shines upon it from the direction of the eye, it will pass through it and not be fixed on its surface.⁑ Accordingly, there will be no light on the surface of that body facing the eye and sending its form to the eye. On the other hand, if that light-source whose light shines upon that transparent body faces the eye, its light will pass through the transparent body and will reach the eye, but it will carry with it no color from the transparent body to the eye, for a transparent body that is absolutely transparent has no color. From that direction, then, sight will perceive the light-source from which the light shines upon the transparent body from behind it, but it will not perceive the transparent body [itself] insofar as sight does not perceive any visible object that is absolutely transparent. Furthermore, if the transparency of the body is the same as the transparency of air, that body will be disposed just like the air, so it will not be perceived by sight, just as air and transparent bodies whose transparency is no less absolute than the transparency of air will not be perceived by sight, for there is no form extending from them to the eye that can affect sight. And the same will hold if some transparent body other than air intervenes between the eye and the visible object and the transparency of the visible object is no less attenuated than the transparency of the intervening body.

8.10 ◉ Et cum res visa fuerit densa, erit colorata, et cum super ipsam oritur lux, figetur in sua superficie, et erit ex colore eius et ex luce que oritur super ipsam forma que extenditur in aere et in corporibus diafonis. Et cum ista forma pervenerit ad visum, operabitur in eo, et ex ea sentiet visus rem visam. Et cum res visa fuerit diafona, sed minus quam aer, habebit colorem secundum suam spissitudinem, et cum super ipsam oritur lux, figetur in ea aliqua fixione secundum illud quod est in ea de spissitudine, et pertransibit in ea secundum suam diafonitatem. Et erit ex ea forma in aere secundum colorem et lucem que sunt in sua superficie, et cum illa forma pervenerit ad visum, operabitur in visum, et sentiet visus illam rem visam. Et propter istam causam nichil comprehendit visus ex rebus visibilibus nisi quando fuerit densum aut fuerit in eo aliquid densitatis.

8.10 ◉ And if a visible object is opaque, it will be colored, and when light shines upon it, it will be fixed upon its surface, and a form of its color, as well as of the light shining upon it, will extend through the air and through transparent bodies. And when this form reaches the eye, it will affect it, and from that effect the eye will sense the visible object. Moreover, when the visible object is transparent, but less so than the air, it will possess [some] color according to its opacity, and when light shines upon it, that light will be fixed in it somehow according to the opacity it possesses but will pass through it according to its transparency. There will thus be a form extending from it through the air according to the color and light on its surface, and when that form reaches the eye, it will affect the eye, and the eye will sense that visible object. For this reason sight perceives no visible object unless it is [completely] opaque or unless there is some opacity in it.⁑

8.11 ◉ Iam ergo declarate sunt cause propter quas nichil comprehendit visus nisi quando fuerint aggregate in eo intentiones predicte, et hoc quod declaravimus est illud quod intendimus declarare in isto tractatu.

8.11 ◉ The reasons why sight perceives nothing unless the aforementioned conditions are met as a whole have now been set forth, and what we have explained is what we intended to explain in this book.