Treasury of Optics

12. The centers of the spheres of the entire eye, of the crystalline lens, of both surfaces of the cornea, and of the convex surface of the albugineous humor, are one single point. [Proposition 7, Part 3].

And since it has been declared [6. 8 n] that the center of the corneaThe clear front "window" of the eye. and the center of the anterior surface of the crystalline lensLatin: glacialis; the lens itself, which medieval scholars thought was the primary organ of sight. are both located on this line, and both are deeper within the eye than the center of the uveaThe iris or pigmented layer., it is better that the center of the anterior surface of the crystalline lens be the very same center as that of the cornea. In this way, the centers of all surfaces facing the opening of the uvea The pupil. are one common point. Thus, all lines extending from this center to the surface of the eye will be perpendicularMeeting at a perfect 90-degree angle. to all the surfaces facing the opening. This will be declared later in our discussion on the nature of vision: that the center of the surface of the cornea and the center of the anterior surface of the crystalline lens are one common center. Therefore, the surfaces of the membranes of sight which face the opening of the uvea are spherical surfaces sharing one common central point.

13. Whether the whole eye is in motion or at rest, the position of its parts remains stable. [Proposition 25, Part 3; also 9 n].

And furthermore, because this center is the center of the visible surface of the eye, which is continuous with the surface containing the whole eye, and because the whole eye is round—except for what is missing from the completion of the sphere due to the consolidative fatThe fatty tissues and connective membranes (sclera) that pad the eye within the socket. at the front of the eye—this missing portion does not cause any variation in the motion of the eye, because it does not touch the hollow of the bone The eye socket or orbit.. Therefore, this center shall be the center of the entire eye; it is thus inside the whole eye. Consequently, the center of the surfaces of the membranes of sight facing the opening of the uvea is inside the whole eye.

Therefore, when the eye moves, the point of the eye which is the center of the surfaces of the sight-membranes does not change, nor is its position relative to those surfaces altered, but it maintains its place. For when the eye moves, it moves only as a whole, and the position of the parts of the whole relative to one another does not change during motion. Since this center is on the inside, its position does not change during its motion. Similarly, the position of the membranes does not change relative to the whole eye—that is, during the motion of sight itself. Therefore, the position of this center relative to the surface of the membranes of sight changes neither in motion nor at rest.

And it has already been declared [5 n] that the bending of the nerveThe optic nerve. during the motion of sight and at rest occurs only at the opening of the eye, which is in the hollow of the bone, since it is only at the back of the whole eye. Indeed, the bending of the nerve during motion and rest occurs only behind its center, and the position of the parts of the whole eye relative to one another changes neither in motion nor at rest. Therefore, the position of the centers of the eye's membranes relative to the whole eye does not change, neither in the motion of sight nor at rest. Thus, the line passing through the center does not change its place or position relative to the whole eye, nor relative to its parts—that is, neither in motion nor at rest. And since the position of this line does not change relative to the whole eye nor its parts, the position of this line does

A detailed anatomical cross-section of the human eye. The eye is shown as a series of concentric circles and segments. Labels identify various parts from front to back: "Cornea", "Uvea", "humor aqueus" (aqueous humor, labeled here as albugineous), "Tunica tenuiss." (the very thin membrane surrounding the lens), "humor crystallinus" (the lens), "humor vitreus" (vitreous humor), and "Tunica reti similis" (the retina, or "net-like" membrane). The optic nerve, labeled "NERVVUS OPTICVS.", extends downwards from the rear, composed of "Duræ cerebri membranæ portio" (part of the dura mater) and "Tenuis cerebri membranæ portio" (part of the pia mater). External features include "Musculi oculum moventes" (muscles moving the eye) on the left and "Tunica adharens" (the conjunctiva) on the right.

not change relative to the surface of the circle of consolidationThe junction where the optic nerve meets the eyeball (the optic disc)., nor relative to its circumference. This circle is the extremity of the hollow of the nerve. Therefore, the position of its surface relative to the surface of the hollow of the nerve is a consistent position. And the tilt of the pyramidal part of the nerveThe conical section of the optic nerve as it enters the eye. over the surface of this circle is a consistent tilt: since the position of the crystalline lens relative to this nerve is a consistent position, and the position of the parts of the eye does not change relative to one another.

Therefore, the surface of the hollow of the nerve, from the location of the circumference of the circle of consolidation to the place of the nerve's bending (which is the pyramidal part), does not change its position relative to the whole eye, nor relative to the circle of consolidation. And it has already been declared [5 n] that the position of the line which passes through the centers of all the parts does not change relative to the circle of consolidation, and that it extends through the middle of the hollow of the nerve. And since the position of this line does not change relative to the circle of consolidation, nor does the surface of the hollow of the nerve (from the circumference of the circle of consolidation to the place of bending) change its position relative to the circle of consolidation: therefore, this line does not change its position relative to the hollow of the nerve until it reaches the place of bending.

Thus, the line that passes through the centers of the membranes passes through the center of consolidation; it stands upon it at right anglesLatin: angulos rectos; perfectly perpendicular. and extends through the middle of the hollow of the pyramidal nerve until it reaches the place where the nerve bends. Its position relative to the surface of the hollow of the nerve (which is inside the whole eye), and relative to all parts of the eye, and relative to all surfaces of the membranes of sight, will always be the same position, and it does not change, neither in the motion of sight, nor in...