Treasury of Optics

ALHAZEN

parts on the surface of the eye. Therefore, the eye will comprehend them mixed together, or it will comprehend nothing of them at all. But if it does comprehend them mixed, the parts or the colors of the parts will not be distinguished or ordered by it. And if it comprehends nothing of these forms, it will comprehend nothing of these parts; and if it comprehends nothing of the parts, it will comprehend nothing of the viewed object. However, the eye does comprehend the viewed object opposite to it that is illuminated, and it comprehends the parts of it of different color, ordered and distinct. And since this is so, it is clear that either the quality of vision will be in another way, or this mode will be part of the proposed mode of seeing.

15. The eye sees individual points of a visible object from individual points of its own surface. 17. 18 p 3.

We must therefore consider whether this mode can agree with the conditions through which the colors of visible things are distinguished, their parts are ordered to the eye, and they come to their existence in the body. We say, therefore, that when the eye is opposite to any visible object, the form of the color and light that are in it will come from every point of the surface of the viewed object to the whole surface of the eye. And from every point of any of the visible things opposite the eye in that arrangement, the forms of color and light that are in them will also come to the whole surface of the eye. If, therefore, the eye were to sense from its entire surface the forms of color and light that come from some one point of the surface of the viewed object, it would sense from its entire surface the form of every point of the surface of the viewed object, and the form of every point of the surfaces of all visible things opposite to it in that arrangement. And thus, the parts of one viewed object would not be ordered by it, nor would they be distinguished by it. But if it senses the form coming from one point of the surface of the viewed object to the whole surface of the eye from only one point of the surface of the eye itself, and does not sense the form of that point with its whole surface, then the parts of the viewed object will be ordered by it, and all visible things opposite will be distinguished. For when it comprehends the color of one point from only one point of its surface, it will comprehend the color of one part of the viewed object from one part of its surface, and it will comprehend the color of another part from another part of its surface, and it will comprehend each part of the visible things from a location on its surface that is different and opposite to it, through which it comprehends another visible object. Therefore, the visible things will be ordered and distinct to it, and likewise the parts of each of them.

16. The crystalline humor is the principal organ of the faculty of vision. 4. 18 p 3.

Let us now consider whether this is consistent and possible in reality. And let us say first that vision is only through the glaciale crystalline humor, whether vision occurs through forms coming from the viewed object to the eye, or according to another mode. However, vision is not through one of the other tunics preceding it, since those tunics are only an instrument of the glaciale. For if an injury happens to the crystalline humor while the other tunics remain healthy, vision is destroyed; and if corruption happens to the remaining tunics, while their transparency remains with the health of the glaciale, the eye will not be corrupted. And even if there is an obstruction in the hole of the uvea the iris/pupil, and the transparency of its humor is destroyed, vision will be destroyed while the cornea remains healthy, and if the obstruction is removed, vision will return. And similarly, if a thick, non-transparent part arrives inside the aqueous humor, and it is on the face of the crystalline humor and between it and the hole of the uvea, vision will be destroyed. And when that thick matter is removed, or turned aside by the aversion of the straight line that is between the glaciale and the hole of the uvea to some part, vision will return. And medicine bears witness to all these things. Therefore, the destruction of the sense of vision occurs upon the corruption of the glaciale while the tunics preceding it are healthy. And that is an argument that the sense of vision is only through this humor, not through the remaining tunics preceding it. And the destruction of the sense upon the destruction of the transparency that is between the glaciale and the surface of the eye by a dense, non-translucent body signifies that the transparency of these tunics is only so that the transparency of the tunics of the eye may be continued with the transparency of the air, and that the bodies which are between the glaciale and the viewed object are made transparent by a continuity of transparency. And the destruction of the sense upon the destruction of the lines that are between the glaciale and the surface of the eye signifies that the sense of the glaciale will only be from the straight lines that are between it and the surface of the eye. We will say, therefore, if the sense of vision is from the color of the viewed object and the light that are in it, and from the form coming from the viewed objects to the surface of the eye, and the sense is only through the glaciale, then the eye will not sense this form through the surface of the eye, but after it has passed through the surface of the eye and arrived at the glaciale. And the form that comes from the viewed object to the surface of the eye passes through the transparency of the tunics of the eye; for it is a property of transparency that the forms of light and color pass through it and extend straightly. And we have already declared this in the air [14 n.]. And when all transparent bodies have been experimented upon, it will be found that light extends in them according to straight lines; and we will declare later, during our discussion on obliquity, how this is to be tested. If, therefore, the sense of vision of light and color that are in the viewed object is from the form coming to the eye from the viewed object, the sense will occur upon the arrival of the form itself to the glaciale. And it has already been declared before [15 n.] that it is not possible for the eye to comprehend the viewed object according to its existence, except when it comprehends the form of one point of the viewed object from only one point of its surface. It is therefore not possible for the glaciale to comprehend the viewed object according to its existence, except when it comprehends the color of one point of the viewed object from a form coming to it from only one point of the surface of the eye; yet the form comes from every point of the surface of the viewed object and passes through the whole surface of the eye to the interior. If, however, from that which comes from one point of the viewed object to the whole surface of the eye, and passes through the tunics of the eye, and arrives at the glaciale, the glaciale comprehends nothing except what comes to it from only one point of the surface of the eye, and senses the color of that point only from the surface of the eye, and arrives at only one point