Human eye. On making an experiment on how the visual power uses the instrument of the eye.

o p - r - S t

Tint the larger glass on the outside and you will make the uvea.

To make an experiment on how the visual power receives the images of objects from the eye, its instrument, a glass ball of five-eighths of a braccio in diameter shall be made. Then let so much be cut from one side that you can put your face into it up to the ears, and then let there be fixed inside at the bottom a box-bottom of a third of a braccio, having in the middle a hole four times larger than the pupil of the eye, or thereabouts, which does not matter. Besides this, let there be fixed a thin glass ball of the size of a sixth of a braccio in diameter; and this done, fill everything with warm clear water, and put your face into this water and look into the ball, and note, and you will see: such an instrument will send the images of S t to the eye, as the eye sends them to the visual power.

h - m - x o y - z - t v - q r - d K - e n - a b c

Here it is posited that the visual power is at the extremity of the optic nerves, of which h m is one. Therefore we shall say that the visual power m cannot see the object a placed on its left side unless it makes the ray of the image of that object pass through the centres of two spheres, namely of the sphere of the light d K e, and through the sphere of the vitreous humour x y t v; and so the path of the ray will be a e r, v z x. Therefore the visual power m will see the object a placed on the left represented to it at x on the left, and so the instrument of the eye cannot render that object on the left in its own site except by way of two intersections, which pass through the axis of the eye, as has been demonstrated.

The pupil is black, because the uvea, which is black, is mirrored in the crystalline sphere, which is in the middle of the albugineous sphere; and it appears blacker still because the light of the air cannot illuminate the albugineous sphere through so narrow an aperture as is that of the pupil of the eye.

c - a r d S b - e o f - t v - p q - n - K h - L

Experience gives that p q is one-third of t v. And because p q receives what is given to it by t v, insofar as t v narrows, the angle o descends further.

How the images are given to the visual power with two intersections of necessity.

The object a sends its likeness to the visual power along the line a r to the part r of the light c d f, then enters through the pupil o and makes the intersection at o, and passes to the vitreous sphere at v, and penetrates that sphere with the part v q, and passes through the intersection n, and terminates at K on the front of the optic nerve K h L, from which it is then referred to the common sense.

Here the adversary says that the intersection n does not occur, but that of the optic nerves serves, and that the pyramid n is cut at the front of the optic nerve, where small things are made large.

r - L - f m - x z e v y - t x d n - o c - b h - a S g

The vitreous sphere is placed in the middle of the eye to set upright the images that intersect within the aperture called the pupil, so that the right returns right and the left returns left in the second intersection, which is made at the centre of that vitreous sphere.