Treasury of Optics

...parts on the surface of sight. Therefore, sight will either perceive them mixed together, or it will perceive nothing from them. If it perceives them mixed together, the parts or the colors of the parts will not be distinguished or ordered by it. And if it perceives nothing from these forms, it will perceive nothing from these parts; and if it perceives nothing from the parts, it will perceive nothing from the visible thing. But sight does perceive the visible thing that is illuminated and opposite to it, and it perceives its parts of different colors as ordered and distinct. Since this is the case, it is established that either the nature of vision occurs in another way, or this mode is but a part of the proposed manner of seeing.

15. Sight sees individual visible points from individual points of its surface. Original: 17. 18 p 3 refers to Proposition 17 and 18 of Book 3.

We must therefore consider whether this mode can agree with the conditions by which the colors of visible things are distinguished, and their parts are ordered within the sight, and agree with their existence in the body. We say, then, that when the sight is opposite to any visible thing, the form of the color and light within it comes from every point of the surface of the visible object to the entire surface of the sight; and from every point of any visible thing opposite to the sight in that arrangement, the forms of color and light within them also come to the entire surface of the sight. If, therefore, the sight sensed from its entire surface the forms of color and light coming from any single point of the surface of the visible object, it would sense from its entire surface the form of every point of the surface of the visible object, and the form of every point of the surfaces of all visible things opposite to it in that arrangement. Thus, the parts of a single visible thing would not be ordered by it, nor would they be distinguished.

However, if it senses the form coming from one point of the surface of the visible object to the entire surface of the sight at only one point of the surface of the sight itself, and does not sense the form of that point across its entire surface: then the parts of the visible object will be ordered by it, and all the opposite visible things will be distinguished. For when it perceives the color of one point from only one point of its surface, it perceives the color of one part of the visible object from one part of its own surface, and perceives the color of another part from another part of its surface. It perceives each part of the visible things from a location on its surface that is different and opposite to that through which it perceives another visible thing. Therefore, the visible things will be ordered and distinct to it, as will the parts of any of them.

16. The crystalline humor is the principal organ of the optical faculty. Original: 4. 18 p 3.

Now, let us consider whether this is suitable and possible to exist. Let us say first that vision occurs only through the glacial humorLatin: glacialis; also known as the crystalline humor or the lens of the eye. Alhazen identifies this as the essential sensory part of the eye., whether vision occurs through forms coming from the visible thing to the sight, or according to another mode. Vision does not occur through any of the other membranesLatin: tunicarum; referring to the layers of the eye like the cornea or uvea. preceding it, since those membranes are nothing but instruments for the glacial humor. For if injury happens to the glacial humor while the other membranes remain healthy, vision is destroyed; and if corruption happens to the remaining membranes—provided their transparencyLatin: diaphanitate. remains along with the health of the glacial humor—sight is not destroyed.

Also, if there is a blockage in the opening of the uveaLatin: foramine uueæ; the pupil. and the transparency of its humor is destroyed, sight is destroyed even if the corneaLatin: corneæ. is healthy; and if the blockage is removed, sight returns. Likewise, if a thick, non-transparent part enters the albuminous humorLatin: humor albugineum; the aqueous humor, which fills the space between the cornea and the lens. and sits in front of the glacial humor, between it and the opening of the uvea, vision is destroyed; and when that thick substance is removed or moved away from the straight line between the glacial humor and the opening of the uvea, sight returns.

All of these things are attested by medicine. The destruction of the sense of sight occurs with the corruption of the glacial humor even when the membranes preceding it are healthy. This is an argument that the sense of sight exists only through this humor, not through the remaining membranes preceding it. And the destruction of the sense upon the destruction of transparency—which is between the glacial humor and the surface of the sight—by a dense, non-translucent body signifies that the transparency of these membranes exists only so that the transparency of the membranes of the eye may be continuous with the transparency of the air, and that the bodies between the glacial humor and the visible thing may be made transparent by a continuity of transparency.

The destruction of the sense upon the destruction of the lines between the glacial humor and the surface of the sight signifies that the sensation of the glacial humor occurs only from the straight lines between it and the surface of sight. We say, therefore, that if the sense of sight is from the color and light of the visible object, and from the form coming from the visible things to the surface of the sight, and the sensation is only through the glacial humor, then the sight does not sense that form through the surface of the eye, but after it has passed through the surface of the eye and reached the glacial humor. And the form that comes from the visible thing to the surface of the sight passes through the transparency of the eye's membranes; for it is a property of transparency that forms of light and color pass through it and extend in straight lines.

I have already declared this concerning air Referring to note 14.. And when all transparent bodies are tested, it will be found that light extends in them along straight lines; we shall declare later in our discourse on refractionLatin: obliquatione; the bending of light as it passes from one medium to another. how this is to be tested. If, then, the sight's sensation of the light and color in a visible object is from the form coming to the sight from that object, the sensation will occur upon the arrival of that form at the glacial humor. And it has already been declared previously Referring to note 15. that it is not possible for the sight to perceive a visible thing as it truly is unless it perceives the form of one point of the visible thing from only one point of its own surface. It is therefore not possible for the glacial humor to perceive a visible thing as it truly is unless it perceives the color of one point of the visible thing from a form coming to it from only one point of the surface of the sight. The form, however, comes from every point of the surface of the visible object and passes through the entire surface of the sight to the interior. If, however, from that which comes from one point of the visible thing to the entire surface of the eye and passes through the eye's membranes and reaches the glacial humor, the glacial humor perceives only what comes to it from one point of the surface of the eye, and senses the color of that point only from the surface of the eye, and it reaches only one point...