Treasury of Optics

OPTICS, BOOK I.

And because the center of sight is the same as the center of the crystalline surface The "crystalline surface" refers to the front of the lens of the eye, which medieval scholars called the glacialis or "icy" humor., [by Proposition 12] all these lines will be perpendicular to the surface of the eye, the surface of the crystalline lens, and all the parallel surfaces of sight. Furthermore, a continuous pyramid pyramis|a cone or pyramid of light rays will be formed upon all these perpendiculars, containing all these lines and the air through which the form In this context, "form" refers to the physical likeness or information of color and light emanating from an object. extends from the entire surface of that visible object opposite the eye, following the lines of direction verticationes|the specific paths or orientations along which light travels of the perpendiculars. The crystalline surface will intersect this pyramid, and thus the form of light and color present on the surface of the visible object reaches that part of the surface which the pyramid encompasses. To any point on this crystalline surface, the form of the corresponding point on the surface of the visible object will arrive along the straight path of the perpendicular line emerging from that point on the object’s surface to the surface of the eye’s tunics The "tunics" are the various layers of the eye, such as the cornea. and the surface of the crystalline lens. It will pass through the transparency diaphanitas|the quality of a medium that allows light to pass through of the eye’s tunics along the straight path of that perpendicular, and no other form will pass through along that same perpendicular path with that form. This form will reach that part of the crystalline lens ordered within it according to the straight lines along which it arrived—lines which are perpendicular to it and converge at the center of sight—just as the parts of the surface of the visible object are ordered.

Furthermore, in that same arrangement, many other forms from many points of the visible surfaces arrive at any single point of this part of the crystalline surface at the same time. Therefore, many forms of many different colors arrive at that part of the crystalline surface defined by the pyramid. If, therefore, the crystalline lens were to sense from the part defined by the pyramid only the form coming to it from the direction verticatio of that pyramid, and did not sense from that part of its surface any other form except the one coming along that direction, it would perceive the form of the object as it truly is, and perceive it ordered according to its actual arrangement. It would also be able to perceive, in that same arrangement, the forms of other visible things besides that specific object, by means of other pyramids defining other parts of its surface; thus, it could perceive the form of any visible thing as it truly is, and perceive their positions relative to one another as they truly are.

However, if the crystalline lens were to sense forms coming to it from refracted refractis|bent as they pass from one medium (like air) to another (like the cornea) directions, it would perceive in that same part—the part defined by that pyramid—a mixture of forms consisting of the parts of that visible object and the forms of many different visible things. These would be mixed from many different colors. It would then perceive from every part of its surface, besides that one part, a form mixed from the forms of many different things. In this way, it would not perceive the form coming along the pyramid's direction as it truly is, nor any form coming along the perpendicular as it truly is, nor any form coming from refracted directions. Consequently, it would not perceive the form of a single visible thing as it is, nor would the visible things opposite it be distinguished from one another at the same time. Yet, sight does perceive visible things as distinct, it perceives the parts of a single visible thing ordered as they are on the object's surface, and it perceives multiple visible things simultaneously.

Since vision occurs from the forms coming from visible things to the eye [by Proposition 14], the crystalline lens must sense nothing of the forms of visible things from the refracted directions. If it did, none of the forms reaching the crystalline surface would be ordered upon it as they truly are; nor would any of the forms of the parts of a single visible object reaching the crystalline surface be ordered according to their true arrangement on the object's surface, unless they reach it solely along the straight path of the perpendiculars raised above the surface of sight.

Moreover, the positions of refracted forms at the surface of the eye arrive at the crystalline surface inverted Original: conuersi. Alhazen is pointing out that refraction would flip the image, much like a modern camera lens, which would confuse the sensory organ if it didn't filter for perpendicular rays., and furthermore, the form of a single point would reach a whole portion of the crystalline surface, not just a single point. This is because the form of a point to the right of the eye, when it extends to a point on the eye's surface at an oblique angle, is refracted toward the left away from the perpendicular that extends from the center of sight to that point on the surface. Thus, the form refracted from the end of the perpendicular arrives in this manner at a point to the left of the point on the crystalline surface where the perpendicular intersects it. Similarly, the form of a point to the left of the eye, extending to that same point on the surface of the eye at an angle, is refracted to a point to the right of the perpendicular and the point on the crystalline surface where that perpendicular lies. This is because refracted forms do not approach the perpendicular emerging from the point of refraction after they have been refracted; they do not arrive by following the perpendicular, nor do they cross over it or precede it after refraction, for this is the nature of refracted forms.

In the same way, the forms of two points located on the same side of the eye, which go to a single point on the surface of the eye and incline toward it on the same side, arrive at the crystalline surface inverted. This is because the two lines along which the two forms of the points extend intersect at the point on the surface of the eye where the two forms meet, and they meet the perpendicular emerging at that point of the eye's surface at that same point. Therefore, when these two lines are inclined from the surface of the eye on the same side of the perpendicular emerging from the center of sight to that point, the forms of the two points are refracted to the side opposite that direction. Also, because the two lines along which the two forms extend to one point on the eye's surface intersect at that point, it is necessary—when they extend in a straight path after that intersection—that their positions appear inverted relative to how they are on the visible object and relative to the perpendicular. The line that was on the right before reaching the surface of the eye becomes the left after passing into the surface, and the left becomes the right. The same will be true of the positions of the two lines along which the two forms are refracted from a single point on the surface of the eye, because both forms refracted from one point approach the perpendicular, and the form that was on the line further from the perpen-