Treasury of Optics

which depend on light and color, except through the reception of the forms of colors and the conveying of them to the opposite parts. [19 n] It has been demonstrated that when the sight is positioned opposite a visible object, the forms of light and color present in that object are conveyed to the sight and reach the surface of the sensing organ. Sight, therefore, does not perceive the light and color of a visible object except through a form extending through the transparent body diaphanum|a medium like air, water, or glass that allows light to pass through located between the object and the eye, and through the effect which the visible object produces upon the sight when it faces it, mediated by that transparent body.

23. Vision does not occur by rays emitted from the sight. Section 3.

And we might say that the transparent body receives something from the sight and returns it to the visible object, and through the continuity of this thing between the eye and the object, sensation occurs. This is the opinion of those who propose that rays exit from the eye This refers to the "extramission theory" held by thinkers like Euclid and Ptolemy, which Alhazen is systematically debunking here.. Let us suppose, then, that it is so: that rays exit from the eye, pass through the transparent body, reach the visible object, and that sensation occurs through these rays. If sensation happens this way, I ask whether something is returned to the sight by these rays, or not. If sensation occurs through rays but they return nothing to the sight, the sight would not perceive; but the sight does perceive the object, and only by means of the rays. Therefore, these rays, which perceive the object, must return something to the sight by which the eye perceives the object. And since the rays return something to the eye by which it perceives, the sight only perceives the light and color of the object because of something coming from that light and color to the eye, delivered by those rays. Thus, under all conditions, vision only occurs through the arrival of something from the visible object, whether rays exit or not. [22 n] It has already been demonstrated that vision is not completed except through the transparency of the intermediate body between the eye and the object, and it is not completed when the intermediate body is not transparent. It is clear that a transparent body is distinguished from an opaque one only in the manner previously described. Since this is as we have said, it is demonstrated that the forms of light and color in the object reach the sight when they are opposite it. Therefore, that which comes from the object to the eye—by which the eye understands light and colors—is nothing other than this form, whether rays exit or not. [14.18 n] It has also been shown that forms of light and color are always generated in the air and in all transparent bodies, and they always extend through the air and transparent bodies in all directions, whether an eye is present or not. Therefore, the exit of rays is redundant and useless. Sight does not perceive the light and color of a visible object except from a form coming from that light and color. [19 n] It has been demonstrated that the form of every point of a visible object opposite the eye reaches the eye along many different lines of direction verticationes|straight paths or lines of radiation, and that the sight cannot perceive the form of an object according to its proper arrangement on its surface unless the reception of forms occurs along straight lines that are perpendicular to the surface of the eye and the sensing member. These perpendicular straight lines only exist when the centers of these surfaces share a single point. Since all this is as stated, it is necessary that the center of the "glacial" surface superficiei glacialis|the front surface of the crystalline lens, which medieval scholars believed was the primary organ where sensation happened and the center of the surface of the eye be one and the same point. Sight, therefore, can comprehend nothing of the forms of visible things except through the directions of straight lines whose endpoints meet at this center alone. This is what we promised to demonstrate earlier in this chapter in the preceding discussion [12 n] regarding the form of the eye: namely, that the center of the lens and the center of the eye's surface are the same common point. Now that this has been demonstrated, it remains to consider the opinion of those who propose that rays exit the eye, and to show what in that view is false and what is true. Let us say, then, if vision occurs by something exiting the eye toward the object: that thing is either a physical body or not a body. If it is a body, when we look at the sky and see the stars, a body would have to exit our eye at that moment and fill the entire space between heaven and earth without the eye being diminished; and this is false. Therefore, vision does not occur through a body exiting the eye toward the object. If that which exits the eye is not a body, it will not perceive the object, for sensation exists only in bodies. Therefore, nothing exits from the eye to the visible object that "senses" that object. It is manifest that vision happens through the sight; yet if the sight does not comprehend the object unless something exits from it, and that which exits does not sense the object, then that which exits the eye returns nothing to the eye by which it might comprehend the object. This "exit" from the eye is not something perceptible, but merely an opinion, and nothing should be believed unless supported by reason. Those who propose that rays exit from the eye believe this because they found that the eye perceives an object even when there is a distance between them; it is a great difficulty for people to imagine Alhazen is acknowledging the psychological hurdle of accepting "action at a distance" without physical contact. that sensation can occur without contact. Therefore, they thought that vision only happens because something exits the eye to sense the object in its place, or to receive something from the object and bring it back to the eye. But because a body capable of sensing cannot exit the eye, and nothing senses a visible object except a body, the only remaining opinion was that what exits the eye receives something from the object and returns it. But it has been demonstrated [14.18.19 n] that air and transparent bodies receive the form of the visible object and return it to the eye and to everything opposite the object. Thus, that which is thought to return something from the object to the eye is nothing other than the air and the transparent bodies between the eye and the object. Since air and transparent bodies return this information to the eye at all times and under all conditions whenever the eye is opposite the object, there is no need for anything to exit the eye. The reasoning that led proponents of rays to claim they exist is therefore redundant; their opinion was based on the idea that vision cannot be completed without something extended between the eye and the object to return information. Since air and transparent bodies do this without needing anything to exit the eye—and are already extended between the eye and the object naturally—there is no reason to propose another thing to return information from the object to the eye.