Treasury of Optics

...and less: and since all lights are of the same kind, and the action of stronger lights is of the nature of pain, then all actions of lights are of the nature of pain. They do not differ except by degree, according to "more" or "less." Because the action of weak, tempered lights upon the sight is slight, the fact that they induce pain is hidden from the senses. Therefore, the sensation of the glacialis|the crystalline humor or lens, then thought to be the primary organ of sight resulting from the action of light is of the nature of a painful sensation. Next, this sensation, which falls upon the crystalline humor, is extended through the optic nerve and reaches the front of the brain. There is the "final sense" and the sentiens ultimum|the ultimate sensor, which is the sensitive faculty located in the front of the brain. This faculty perceives the things sensed. The sight itself is nothing but an instrument of this faculty, since the sight receives the forms of things seen and gives them back to the ultimate sensor; the ultimate sensor then perceives these forms and understands the visible things contained within them. That form on the surface of the crystalline humor is extended through the body of the crystalline humor, then into the subtle body the "pneuma" or animal spirit which is in the hollow of the nerve, until it reaches the neruum communem|the optic chiasm, where the two optic nerves meet. Vision is completed at the arrival of the form at this common nerve; from the form arriving at the common nerve, the ultimate sensor perceives the forms of the things seen.

The observer perceives visible things with two eyes, and thus it must be that the form of the thing seen reaches each eye. Therefore, two forms reach the sight from a single visible thing, yet the observer perceives only one visible thing. This is because the two forms that reach the two eyes from one object, when they reach the common nerve, run together and are superimposed one upon the other to become a single form. From that unified form made from two, the ultimate sensor perceives the form of that visible object. The proof of this is that the two forms reaching the two eyes from one object are arranged and made into one form before the ultimate sensor perceives them. That the ultimate sensor does not perceive the form except after the union of the two forms is shown by this: when an observer changes the position of one eye while the other remains unmoved—and the motion of the eye shifted in position is toward the front—he will see two of the single thing facing him. Yet if he opens one eye and covers the other, he will see only one. Therefore, if the sensor perceived the object as one simply because it is one, it should always perceive it as one; and if two forms always reached it from one object, it would always perceive one thing as two. Since the ultimate sensor does not perceive a visible thing except from the form reaching it, and sometimes it perceives one visible thing as two, and sometimes as one, it is a sign that what reaches it when it perceives two is a double form, and when it perceives one, what reaches it is a single form. Since in both arrangements two forms reach the two eyes from one visible object, and yet what is returned to the ultimate sensor is sometimes a double form and sometimes a single one—and since the form returned to the ultimate sensor is only returned from the sight—then that which is returned to the ultimate sensor from the two forms reaching the two eyes from one object, when it perceives it as one, is a single form. Since this is so, the two aforementioned forms are extended from the two eyes and meet before the ultimate sensor perceives them; after they meet, the ultimate sensor perceives the form united from them. But the two forms that reach the two eyes from one object when the ultimate sensor perceives it as two are extended from the two eyes but do not meet; they reach the ultimate sensor as two forms. The perception of one visible thing, which appears sometimes as one and sometimes as two, signifies that vision is not through the eye alone. If it were, then when perceiving a visible thing that appears as one, the two eyes would perceive one and the same form from the two forms reaching them; if this were the case, they would always perceive one form from two. Since one visible thing is perceived sometimes as one and sometimes as two, and in both arrangements there are two forms in the two eyes, it is signified that there is another sensor besides the two eyes to which the two forms from one object reach—appearing as one when they are united, and as two when they are perceived as two. This shows that the sense is not completed except through that sensor alone, not through the eye alone. Furthermore, the sense does not extend from the members to the ultimate sensor except through nerves connected to the members and the brain. Therefore, the two forms extend from the eye in the nerve stretched between the eye and the brain until they reach the ultimate sensor. These two forms extend from the two eyes and meet at the place where the two nerves join. The clear proof that the forms of things extend through the hollow of the nerve and reach the ultimate sensor—and that vision is completed after this arrival—is this: when there is an oppilatio|obstruction or blockage in that nerve, vision is destroyed; and when the obstruction is removed, vision returns. The medical art testifies to this. Why the two forms sometimes meet and sometimes do not is because when the position of the two eyes is natural, their position relative to one object will be a similar position; thus the form of one visible object reaches two places of similar position. When the position of one eye is deflected, the position of the eyes relative to that object will differ, and thus two forms of that object will reach different positions. It has already been said regarding the form of the eye [4 n] that the position of the common nerve from the two eyes is a similar position; thus the position of two similar places from the two eyes will be at the same place in the common nerve, and from the two hollow nerves one is made, in which the two forms of sight are united. One might say that the forms coming to the eye do not reach the common nerve, but that the sensation is extended from the eye to the common nerve just as the sense of pain and touch is extended, and then the ultimate sensor perceives that sensible thing. We say that the sensation itself coming to the eye reaches the common nerve entirely; however, the sensation reaching the eye is not merely a sensation of pain, but a sensation of an action of the nature of pain, and it is the sensation of light and color, and the sensation of the arrangement of the parts of the visible object. But the sensation of the diversity of color and the arrangement of parts of the object...