Of the human eye. How the images of any body that pass to the eye through some small aperture imprint themselves upside down in its pupil, and the sense sees them upright.

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

d b - a c

Crystalline sphere placed in the middle of the eye.

Here it is presupposed that the pupil has visual power in every part of its breadth, and this is in fact so, and without this every demonstration collapses.

The pupil of the eye, which through a very small round aperture receives the images of bodies placed beyond that aperture, always receives them upside down, and the visual power always sees them upright as they are. And this arises because the said images pass through the centre of the crystalline sphere placed in the middle of the eye, and in that centre they unite in a point, and then dilate on the opposite surface of that sphere without straying from their straightness, and on that surface the images are set upright according to the object from which they are caused, and from there they are taken up by the receptive faculty and sent to the common sense where they are judged. It is proved to be so. Let a n be the pupil of the eye K h, and let p be the minute round aperture made in the paper with the fine point of a stylus, and let the object placed beyond that aperture be m b. I say that the upper part of that object cannot come to the upper part of the pupil of the eye by the straight line m a, because at v its passage is impeded by the interposition of the paper. But that upper extreme m passes in a straight body through the aperture to the lower part n of the pupil, or you may say of the crystalline sphere, and from there directs its course to the centre of that sphere and rises to the upper part of the opposite side, and from there runs to the common sense, as has been said.

Below is demonstrated the experiment from which the certainty of this new investigation was born. Second.

g d - a n m o p - L K h q - r q S - t v x y c

Let an instrument be found that produces the same effect of necessity, and thus you will have found the true interior figure of the eye.

Let g d be the eye whose pupil a p sees, through the aperture q (made in the paper n S), the object t c. I say that if you move the stylus L q from above to below the pupil, close to that pupil along the line K h, which will appear outside that aperture q, such motion of the stylus is contrary to its true motion. And the reason is that, the stylus touching the line a c, it touches the highest line inside the aperture q and the lowest outside that aperture; and in the descending of the stylus it touches the line n y, that is, the highest inside the aperture and the lowest outside it; and so, continuing the descent inside the aperture before the pupil with your stylus, you follow the contrary outside that aperture, because if you descend inside along the fronts of the lines a n m o p, you rise outside along the fronts of the lines c y x v t. Therefore that stylus which touches the line a c occupies the site c, and if you descend with it to the line n, you occupy outside the aperture the site y, and so it continues to the end of the motion.

How and in what way the pupil takes the images of objects without the interposition of the said aperture.

If in the second demonstration the pupil a p receives at p the upper part of the image t by means of the aperture q, and if you remove the interposition of that aperture, the image t will imprint itself at a and also at p, because every point of the pupil sees the whole image t c.

a - b c d

Every point of the pupil sees the whole object, and every point of the object sees the whole pupil.