Treasury of Optics

ALHAZEN

and this compression is located opposite the opening in the front of the uveaThe uvea here refers to the pigmented layer of the eye, specifically the iris and choroid., and its position corresponds to it. This humor is divided into two parts of different transparency: one part is in the front, and the other in the back. The transparency of the posterior part is like that of broken glass original: "uitri quasi frustati", and this part is called the vitreous humor. These two combined parts are contained by an extremely thin membrane called the araneaLiterally "spider-web"; this refers to the lens capsule or arachnoid membrane., because it resembles the texture of a spider's web. In the posterior part of the hollow of the uvea's sphere, there is a round opening at the end of the nerve's cavity, and the glacial sphereThe crystalline lens, which medieval scientists believed was the primary organ of sight. is set into this opening. The roundness of this opening (which is the end of the nerve) contains the middle of the glacial sphere, and the uvea is joined firmly with the glacial sphere at the circle surrounding that opening. It is said that the uvea originates from the inner layer of the two tunics of the two optic nerves, and the cornea originates from the outer layer of those same nerves. The hollow of the uvea is filled with a white, thin, clear, and transparent humor called the albugineous humorThe aqueous humor; so named because its consistency and appearance resemble egg white (Latin: "albugo")., because it is like the white of an egg in its thinness, whiteness, and transparency. It fills the hollow of the uvea, touches the front of the glacial sphere, fills the opening in the front of the uvea, and touches the hollow of the cornea. The glacial sphere is positioned over the hollow of the nerve, and the vitreous humor follows the hollow of the nerve. Thus, the cornea, the albugineous humor, the glacial sphere, and the vitreous humor follow one after the other. All of these layers are transparent. The opening in the front of the uvea is opposite the opening of the nerve's cavity. It is said that the visual spiritA theoretical substance in medieval medicine thought to flow from the brain to the eyes to enable sight. is sent out from the upper part of the brain, fills the two hollows of the first two nerves joined to the brain, reaches the common nerve The optic chiasm., fills its cavity, comes to the two second optic nerves, fills them, reaches the glacial sphere, and gives it the power of sight.

5. Whether the whole eye is in motion or at rest, the position of its parts remains stable. [Book 3, proposition 25].

Between the circumference of the glacial sphere where it joins the uvea, and the opening in the hollow of the bone from which the nerve emerges, there is a small space. The nerve extends through this space from the end of the opening to the circumference of the glacial sphere, following a pyramid-like expansion; the further it gets from the opening in the bone, the more it expands, until it reaches the circumference of the glacial sphere and is fused with its edge. The body of the consolidativaThe sclera or "firming" layer of the eye. contains this pyramidal part of the nerve and also contains the sphere of the uvea; the sphere of the uvea sits in front of the middle of the consolidativa toward the visible part of the eye. The body of the consolidativa is fused with the uvea and with the pyramidal end of the nerve, maintaining its position. Therefore, when the eye moves, it moves as a single whole, and thus the nerve upon which the eye is built will pivot during its motion. This pivoting occurs at the opening in the hollow of the bone, because the hollow of the bone contains the whole eye, and the eye moves as a whole within that hollow. The consolidativa is fused with the forward part of the eye, the nerve, and the remaining tunics, and it always preserves their relative positions. Thus, the pivoting of the nerve during the eye's movement only occurs from the back of the whole eye, at the opening in the hollow of the bone. Likewise, when the eye is at rest and the nerve is angled, the angle only exists at the opening in the bone. For the position of the parts of the whole eye does not change relative to one another, whether in motion or at rest. Therefore, the bending of the nerve upon which the eye is constructed occurs only at the opening in the hollow of the bone, whether the eye moves or remains still.

6. The whole eye and the sphere of the uvea have different centers; the center of the eye is deeper. [Book 3, proposition 8].

The visible surface of the cornea is a spherical surface, and it is continuous with the surface of the whole eye and with the whole eye itself. The whole eye is larger than the sphere of the uvea, which is a part of it. Since the visible surface of the cornea is part of the surface of the whole eye, it is larger than the surface of the uvea's sphere; therefore, its radius is larger than the radius of the uvea. Because the inner surface of the cornea placed over the opening of the uvea is a concave spherical surface, parallel to its own visible outer surface (since the cornea is of uniform thickness), the center of the concave surface of the cornea is the same as the center of its visible convex surface. But since the concave surface of the cornea intersects the sphere of the uvea at the circumference of the opening in the front of the uvea, its center must be deeper in the back than the center of the uvea; this is according to the properties of the centers of intersecting spheres. Furthermore, because the sphere of the uvea is not in the middle of the consolidativa but is positioned toward the front surface of the eye, and because the visible surface of the eye belongs to a sphere larger than the sphere of the uvea, the center of the visible surface of the eye will be deeper than the center of the uvea.

7. A straight line connecting the centers of the spheres of the cornea and the uvea, when extended, passes through the center of the opening of the uvea and the middle of the optic nerve cavity. [Book 3, proposition 9].

A straight line that connects the two centers—namely, the center of the cornea’s surface and the center of the uvea—when drawn straight, reaches the center of the opening in the front of the uvea and the midpoints of the two parallel surfaces of the cornea. For the concave surface of the cornea and the convex surface of the uvea are spherical surfaces that intersect each other; and the line that connects their centers passes through the center of the circle of their intersection and is perpendicular to its surface. This is because a line that emerges from the center of the circle of intersection and is perpendicular to its surface must pass through the centers of both spheres.