Treasury of Optics

...its motion. Therefore, these are the positions of the membranes of the eye, the positions of their centers, and the position of the straight line passing through their centers. Both eyes are similar in all their dispositions, in their membranes, in the shapes of their membranes, and in the position of each membrane relative to the whole eye. And since this is so, the position of each of the centers (whose distinction has been declared) relative to the whole eye and its parts is like the position of the center corresponding to that center in the other eye relative to that whole eye and its parts. And since the position of the centers in both eyes is a similar position, the position of the line passing through the center in one eye relative to the whole eye, its parts, and its membranes will be similar to the position of the line passing through the center of the other eye relative to the whole eye, its parts, and its membranes. Therefore, the position of the two lines passing through the centers of the membranes of the eye from both eyes is a similar position in all its dispositions. Both of the consolidations referring to the eye's supporting structures are consolidated with them, since from them emerge two small muscles, one of which is in the part of the eye near the tears and the other in the posterior part. And the eyelids and eyelashes contain each eye. Therefore, this which we have declared is the disposition of the composition of the eye, its form, and the form of its membranes. And everything that we have said about the membranes of the eye and their composition has already been declared by anatomists in books of anatomy.

ON THE QUALITY OF VISION, AND THINGS DEPENDENT UPON IT. Chapter 5.

14. Vision occurs through rays flowing from the visible object externally to the eye. 6 p 3.

It has already been declared above in that from any body illuminated with any light, light exits toward every part opposite to it. Therefore, when the eye is opposed to some visible thing, and that thing is illuminated with any light, light will come from the light of the visible thing to the surface of the eye. And it was declared that it is a property of light to act upon the eye, and that it is the nature of the eye to be affected by light. Therefore, it is fitting that the eye should not sense the light of the visible thing except from the light coming from it to the eye. And it was already declared that the form of the color of any colored body illuminated with any light is always associated with the light coming from that body to any part opposite to that body, and light and the form of color will always be together. Therefore, with the light coming to the eye from the light of the visible body, there will always be the form of the color of the visible body. And when light and color come together to the surface of the eye, the eye senses the color that is in the visible thing from the light coming to it from the visible thing. Therefore, it is more fitting that the sense of the eye for the color of the visible thing should be only from the form coming to the eye itself with the light, and the form of color is always mixed with the form of light and is not distinct from it. Therefore, the eye does not sense light unless it is mixed with color. It is more fitting, therefore, that the sense of the eye for the color of the visible thing and the light that is in it should be only from the form mixed with the light and color coming to it from the surface of the visible thing. And again, the membranes of the eye which are situated at the middle of the anterior of the eye are transparent and touch each other, per 4 n and the first of them, namely the cornea, touches the air, in which the form first comes. And it is a property of light to pass through into any transparent body; likewise, it is a property of the form of color, which is associated with light, to pass through into a transparent body, and therefore it extends in transparent air just as light extends. And it is of the nature of transparent bodies to receive the forms of light and color, and to return them to parts opposite to themselves. Therefore, the form coming from the visible thing to the surface of the eye will pass through the transparency of the membranes of the eye, through the foramen that is in the anterior of the uvea, and will therefore arrive at the humor glacialis crystalline humor, and will pass through it according to its transparency. Therefore, it is more fitting that the membranes of the eye should not be transparent unless the forms of light and colors coming to it pass through them. Let us therefore gather what is composed of all these, and say that the eye senses the light and colors which are on the surface of the visible thing, and that they pass through the transparency of the membranes of the eye. And this is that in which the opinion concerning the quality of vision rested. We will therefore say now that the quality of vision is not asserted to be in this way only, since this mode is destroyed unless another thing is added to it. For because the form of light and color of any colored and illuminated thing extends in transparent air continuous with it to all opposite parts, the eye, however, is opposed at the same time to many visible things of different color, and between each of them and the eye there are straight lines in the air with a continuous medium between them. And when the forms of light and color, which are in the visible thing opposite the eye, come to the surface of the eye, the forms of light and color of any visible things opposed to the eye will arrive at the surface of the eye at the same time. And when the forms extend from the visible thing to the eye, they arrive at the whole surface of the eye. And when because of opposition the form, which arrives from the visible thing to any part opposite, does not arrive at the eye—unless it is so when the eye is opposed to some surface of the visible thing, and the form of its color and light arrives at the surface of the eye, and the viewer sees in that time other visible things of different color opposed to the eye—then the form of light and color of any of those visible things will come to the surface of the eye, and the form of all those visible things will arrive at the whole surface of the eye. Therefore, many different lights and many different colors will arrive at the whole surface of the eye, and each of them fills the surface of the eye. Therefore, a form mixed from different colors and different lights will arrive at the surface of the eye. If, therefore, the eye sensed that mixed form, it would sense a color different from the color of any of those things, and the visible things would not be distinguished from it. And if it sensed one of those visible things and did not sense the rest, it would perceive one visible thing and not others. But it perceives all those visible things at the same time, and it perceives them distinctly. And if it did not sense one of those forms, it would sense nothing from them or from other visible things opposed to it; but it does sense everything. And again, there can be different colors in the same visible thing, and from any part of it light and color exit along all straight lines which extend in the continuous air. When, therefore, the parts of one visible thing are of different color, the form of color and light will arrive at the whole surface of the eye from each of them, and thus the colors of those...