Of the human eye.

t u x p h - S - r - a n o p q

altore

adversary

If the whole of the pupil, according to the adversary, does not see, why does it grow at night? Does it grow in order to see a greater sum of things, or to know them better? Without doubt it grows to see them larger, and consequently better, rather than to see many of them. Since it has to judge only one at a time, such growth would not occur [for that purpose].

The pupil of the eye has visual power, whole throughout and whole in every part of it; and a thing set before the eye, smaller than its pupil, does not hide from the eye any other remote object; and although it be dense, it performs the office of a transparent thing. Here the adversary says that the visual power is reduced to a point, and from this it follows that every object set before the pupil that is larger than the point will hide the knowledge of what is behind it. And that it be true that the visual power is reduced to a point: the convexity of the eye, which with its parts turns toward a great part of the universe set before it, could not be of such curvature unless it were equidistant from that point, and on its surface [the lines] would have to cut at equal distance from that point, so that each of them, with the same real proportions, would correspond in the cuttings of the angles to the proportions of the likenesses of the bodies corresponding to that point.

To him the experiment is demonstrated, and then the necessity of that experiment is concluded. And first, for the experiment, let there be set before the pupil the thickness of a medium sewing needle, and let it be as near as possible, and it will be seen that the knowledge of no object placed behind that needle at some distance is impeded. This that I say is wholly confirmed by experiment. And necessity makes it so, because if such visual power were reduced to a point, every smallest object placed before that visual power would hide the knowledge of a great part of the sky; for if a great part of the sky sends the species of its stars to the pupil, an object placed near it, equal to half its diameter, would hide from it almost half of that sky; whence Nature, so that nothing be lacking to the eyes of animals, has made that light have the fewest and smallest impediments possible; of which the reduction of the visual power to a point would be a very great impediment, because, as is said, every smallest object set before it would make a great occupation.

The experiment also shows that the meshes of sieves made of thick horse-hairs, placed before the eyes, hide nothing behind them, and hide the less the nearer they approach the eye; whereas, if the visual power were in a point, the nearer such hairs approached it, the wider the space they would have to hide. Therefore, the experiment giving the contrary, it is true that the visual power is infused throughout the whole pupil, and every part operates in itself and sees behind such hairs, encircling them and seeing beyond the piece of their thickness, and of necessity causes the pyramids near the said hairs.