...its motion. These, therefore, are the positions of the membranesLatin: tunicarum; the layers or "coats" of the eye, such as the cornea and retina. of sight, the positions of their centers, and the position of the straight line passing through their centers. Both eyes are alike in all their arrangements, in their membranes, the shapes of their membranes, and in the position of each membrane with respect to the whole eye. Since this is so, the position of each of the centers—the distinction of which has been declared—within the whole eye and its parts is the same as the position of the corresponding center in the other eye and its parts. And since the position of the centers in both eyes is similar, the position of the line passing through the center in one eye with respect 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 with respect to that whole eye, its parts, and its membranes. Therefore, the position of the two lines passing through the centers of the membranes of sight from both eyes is a similar position in all its arrangements. And each of the sclerasLatin: consolidativarum; the tough white outer layer of the eyeball. is joined with them, since from them emerge two small muscles, one of which is on the side of the tear ducts and the other on the back side. The eyelids and eyelashes contain both eyes. This, then, which we have explained, is the arrangement of the composition of the eye, its form, and the form of its membranes. Everything we have said concerning the membranes of the eye and their composition has already been explained by anatomistsAlhazen is likely referring to Galen and other Greek medical authorities whose works were translated into Arabic. in books of anatomy.

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

14. Vision occurs by rays flowing from the visible object outside to the sight. [Proposition 6, Part 3].

It has already been declared above that from every body illuminated by any light, light goes out toward every part opposite it. Therefore, when the sight is opposite any visible thing, and that thing is illuminated by any light, light will come from the light of the visible thing to the surface of the sight. And it has been declared that it is a property of light to act upon the sight, and that the nature of sight is to be affected by light. It is appropriate, therefore, that the sight should not perceive the light of a visible thing except from the light coming from it to the sight.

It has also been declared that the form of the color of any colored body illuminated by any light is always associated with the light coming from that body toward every part opposite it; light and the form of color will always be together. Therefore, along with the light coming to the sight from the light of the visible body, there will always be the form of the color of the visible body. And since light and color come together to the surface of the sight, the sight perceives the color which is in the visible thing from the light coming to it from that visible thing. It is more appropriate, then, that the sense of sight's perception of the color of a visible thing should only come from the form of color reaching the sight with the light; the form of color is always mixed with the form of light and is not distinct from it.

Sight, therefore, does not perceive light except when mixed with color. It is more fitting that the sight’s perception of the color and light of a visible thing should only be from the form mixed with light and color coming to it from the surface of the visible object. Furthermore, the membranes of sight situated at the middle of the front of the eye are transparentLatin: diaphanae; materials that allow light to pass through them. and in contact with each other, according to proposition 4, note n and the first of these—namely the corneaThe clear front "window" of the eye.—touches the air, where the form first arrives. It is a property of light to pass through any transparent body, and similarly, it is a property of the form of color (which is associated with light) to pass through a transparent body; thus, it extends through the transparent air just as light extends.

It is the nature of transparent bodies to receive the forms of light and color and to pass them on to the parts opposite them. Therefore, the form coming from the visible thing to the surface of sight will pass through the transparency of the membranes of sight, through the opening The pupil. which is in the front of the eye, and will reach the crystalline lensLatin: humorem glacialem; literally "the icy humor." Medieval scholars believed this was the central organ of vision. and pass into it according to its transparency. It is more fitting, then, that the membranes of sight should be transparent only so that the forms of light and colors coming to it may pass through them.

Let us now gather what is composed from all of these points and say that the sight perceives the light and colors which are on the surface of the visible thing, and that they pass through the transparency of the membranes of sight. This was the view in which the natural philosophers’Latin: physicorum; referring to the Aristotelian tradition of "physics" or natural science. opinion on the quality of vision rested.

However, we shall now say that the quality of vision is not asserted to be of this kind alone, because this model is destroyed unless something else is added to it. Since the form of light and color of any colored and illuminated object extends through the transparent air—which is continuous with it—toward all opposite parts, the sight is simultaneously opposite many visible things of different colors. Between each of these and the sight there are straight lines in the air, forming a continuous medium between them. Since the forms of light and color in the visible thing opposite the sight come to the surface of the eye, then the forms of light and color of every visible thing opposite the eye will reach the surface of the eye at the same time.

Since forms extend from a visible thing toward every opposite part and do not reach the eye except by being opposite it, the form that reaches the eye from a visible thing reaches the entire surface of the eye. This being so, when the sight is opposite any surface of a visible thing, and the form of its color and light reaches the surface of the eye, and the observer sees at that time other visible things of different color opposite the eye, then the form of light and color of each of those visible things will come to the surface of the eye, and the form of all those visible things will reach the entire surface of the eye.

Thus, many different lights and many different colors will reach the entire surface of the eye, and each of them fills the surface of the eye. Consequently, a form mixed from different colors and different lights will reach the surface of the eye. If, therefore, the sight perceived that mixed form, it would perceive a color different from the color of any of those things, and the visible objects would not be distinguished by it. If it perceived one of those visible things and not the rest, it would comprehend one visible thing and not others; but in fact, it comprehends all those visible things at the same time, and it comprehends them as distinct. If it did not perceive one of those forms, it would perceive nothing from them or from other visible things opposite it; but it perceives everything. Furthermore, there can be different colors on the same visible object, and from every part of it, light and color go out according to all the straight lines that extend through the continuous air. Therefore, when the parts of a single visible thing are of different colors, the form of color and light from each of those will reach the entire surface of the eye, and thus their colors will be mixed...