Eye of man.

S t - n m - a c - r - b d - g - p q

That it is true that every part of the pupil has visual power, and that such power is not reduced to a point as the perspectivists would have it, that is, that all the images of objects come to the eye by a pyramid and are reduced to an angle in which the judgment of the thing seen is made, here experience shows the contrary. And it is tested thus. Let a hole be made by a thick needle in a paper, about the size of a grain of millet, and let this paper be set before the pupil of the eye at a distance of a third or a quarter of a braccio; and through such hole r look toward the open air p q. Then interpose that needle, or another similar straw, between the pupil of your eye and the said hole of the paper, but make that needle close to the eye, so that it touches the tips of its eyelids. Then move that needle down and up and to the right and to the left, opposite your pupil, and you will plainly see in the air beyond that hole the likeness of that needle make all the motions contrary to what you make with the needle between your eye and the aforesaid hole.

The cause of what is said above happens because the visual power is spread through the whole pupil of the eye, and in every point of it the images of objects set before that eye are known; which, were it not so, this experiment would have no place. It is proved thus. Let the circle S t stand for the pupil of the eye. Let the paper set before it, remote from the eye by 1/3 of a braccio, be a b c d, in which is made the hole the size of a grain of millet, r. Let the thickness of the needle, set as close to the pupil of the eye as possible, be n, which moves opposite the said hole r from n to m with slow motion. Then you will see the likeness of that needle move in the air outside that hole from q to p, that is, in contrary motion; and this arises because at the beginning of such motion the needle, which was above at n, its likeness passes through the hole r and occupies the air on the lower part at q; and when the needle descends from n to m, its likeness in the air rises from q to p, and always the intersection of the straight lines of such image, or you may say the shadow of that needle, will be made at the point g; and so every motion made from any aspect beyond that hole will be reversed; which, if the visual power were not at S, you would not see the image of that needle at q, and if it were not at t, you would not see it at p, and the same is understood in every part of that pupil.

v r x - a n - c g - t - h L - m f

The adversary says that such vision is made in an angle, that is, at a point, and that such point would make the same experiment that the needle made with the hole of the paper, and that it cannot be otherwise, and he says: let the point r be in the pupil v x, let c h g L be the paper in which the hole t is made, let a be the needle that descends to n, and its likeness will be seen to move beyond that hole in contrary motion also, and it will move from f and rise to m.

To this one it is replied that he has not kept in mind the ninth [proposition] of the first [book], in which it was said how every vision made by the eye through a uniform medium is made along a straight line; the needle a set before the point r will occupy, by the straight line r m, the point m; and when it has descended from a to n, it will occupy, by the straight line r f, the point f. Therefore we have concluded our proposition.