OPTICS, BOOK I.

8. The center of the uveal sphere is deeper than the centers of the other parts of the eye. [Proposition 8, Part 3].

And because the concave surface of the corneaThe clear, protective outer layer of the eye. touches the surface of the albugineous humorNow known as the aqueous humor; the clear fluid in the front of the eye., which is in the front opening of the uveaThe pigmented layer of the eye, including the iris., and is placed upon it: the convex surface of the albugineous humor is also a spherical surface, the center of which is the center of the surface placed upon it. Therefore, the visible surface of the cornea, its inner surface, and the convex surface of the albugineous humor (which touches the hollow of the cornea) are all spherical and equidistant surfaces. Their center is therefore a single common point, and it is deeper in the eye than the center of the uvea. A line that passes through the center of the uvea, the center of the cornea, and the center of the opening in the front of the uvea The pupil., when extended straight, will pass through the middle of the hollow of the nerve upon which the eye is composed: because the opening in the front of the uvea is opposite the opening in the back of the uvea, which is the end of the hollow of the nerve [according to note 4].

9. The straight line connecting the centers of the crystalline and uveal spheres, when continued, falls upon the center of the circle joining the crystalline and vitreous spheres with the uvea; and it is perpendicular to it. [Proposition 10, Part 3].

The surface of the front of the glacial sphereThe "glacial" or "crystalline" humor refers to the lens of the eye. is also a spherical surface, and it intersects the sphere of the uvea: its center is therefore deeper within the eye than the center of the uvea. The straight line that connects their centers passes through the center of the circle of intersection and is perpendicular to it. This circle of intersection between the front surface of the lens and the surface of the uveal sphere is either the circle marking the boundary of the attachment between the lens and the uvea, or it is parallel to it. Because the surface of the front of the lens is opposite the opening in the front of the uvea, its position is consistent with it. Therefore, the edge of this surface (which is the circle of intersection between the two surfaces of the lens and the uvea) is either the circle of attachment itself, or is parallel to it.

If the circle of intersection between the two surfaces of the lens is the circle of attachment, then this circle is the intersection between the front surface of the lens and the surface of the uvea. And if the circle of intersection between the two surfaces of the lens is parallel to the circle of attachment of the lens sphere with the uvea (which happens if the attachment is in the posterior part of the lens), then the surface of the front part of the lens—if mentally extended across its own sphere—will intersect the sphere of the uvea at a circle parallel to this circle (namely, the circle of intersection between the two surfaces of the lens), due to the similarity of the position of this circle to the circumference of the uveal sphere.

And because this circle is parallel to the circle of attachment, the circle of intersection between the front surface of the lens and the uveal sphere will be either the circle of attachment itself or parallel to it. If this circle is the circle of attachment itself, the straight line passing through the center of the front of the lens and the center of the uvea will pass through the center of that circle and be perpendicular to it, since that circle is the intersection between those two spherical surfaces. But if this circle is parallel to the circle of attachment, and parallel to the circle of intersection between the two surfaces of the lens, then it exists with the circle of intersection between the two surfaces of the lens on one spherical surface (which is the surface of the front of the lens). Therefore, the line passing through the center of the uvea and the center of the front surface of the lens passes through the center of the circle of attachment in all arrangements, and is perpendicular to it, whether the circle of attachment is the circle of intersection between the front of the lens and the uveal sphere, or is parallel to that circle.

10. The center of the crystalline sphere is higher than the center of the vitreous sphere. [Proposition 11, Part 3].

Again, the surface of the front of the lens and the surface of the remainder of the lens The back part of the lens. are two spherical surfaces intersecting each other; therefore, the center of the front surface is shallower original: "altius," meaning closer to the front of the eye. than the center of the back surface.

11. The straight line connecting the centers of the spheres and the uvea, when continued, falls into the center of the vitreous and the middle of the optic nerve cavity. [Proposition 12, Part 3].

The straight line connecting these two centers passes through the center of the circle of intersection and is perpendicular to it; and it has already been shown [note 9] that it passes through the center of the circle of attachment and is perpendicular to this circle, and passes through the center of the rest of the lens. Thus, the line that passes through the center of the uvea, the center of the front of the lens, and the center of the circle of attachment, and is perpendicular to this circle, passes through the center of the remainder of the lens.

And since this line passes through the center of the remainder of the lens and the center of the circle of attachment, and is set upon the circle of attachment at right angles, it extends through the middle of the hollow of the nerve upon which the eye is composed, because the circle of attachment is at the extremity of the nerve's hollow. And it has already been shown [note 7] that the line passing through the center of the uvea, the center of the cornea, and the center of the opening in the front of the uvea, extends through the middle of the nerve's hollow.

Therefore, this line, which passes through the two centers of the lens surface and the center of the uvea, is the same line that passes through the center of the cornea and the center of the opening in the front of the uvea. This line, then, passes through the center of the cornea, the center of the uvea, the two centers of the lens surface, the center of the opening in the front of the uvea, and the center of the circle of attachment. It passes through the middle of all the layers opposite the openings of the uvea. It is perpendicular to the surfaces of all the layers opposite the opening of the uvea, perpendicular to the surface of the opening of the uvea, perpendicular to the surface of the circle of attachment, and extends through the middle of the hollow of the nerve upon which the eye is composed.