a-c-b; m - n - o p. If the eye looks straight ahead of itself along a straight line to the point c, it will see the movements made along the line of the ears in a distant location, at the points a b. It is therefore necessary that the lines be broken, changing as they pass from the rareness of the air to the density of the humour of the eye, as is shown at m n o p. If the visual power were produced at the centre of the eye, all things seen would appear of equal size, because every centre is indivisible, and each image by itself - and many together arriving at that point - would be indivisible. All indivisibles being of equal quality, everything seen would appear of equal size.

All the images of objects that have to pass from one surface to another of spherical and transparent bodies must make their way through the centre of that body, and it is necessary that the images be turned upside down. The centre being indivisible, no thing however small can pass through it without becoming indivisible there and without falling into the intersection. Therefore, since the surface of the eye gives to the receptive faculty the images sent to it by the opposing objects by means of a spherical body set in the middle of the eye, it is necessary that the images make an intersection and turn upside down at the point of contact with the receptive faculty. And this reasoning seems to confirm that there was another intersection before the spherical body, which first turned the images upside down, while the second intersection, of the spherical body, turns them back from below to above; and in this way the receptive faculty sees them upright. If you place a glass ball full of water before your eyes, all the images of the objects that pass within it will appear to you upside down. And if you place two of them, one behind the other, the images in the first, toward the eye, will appear set upright again in their natural direction. And it seems, by reason, that there is no thing so subtle that, placing it near the point of the eye, would not block out the image of every large object. Nevertheless we see that placing something as thick as the pupil, touching the lids of the eye, does not block out anything set opposite the eye. If you take half a glass ball and put your face inside it and seal it well at the join of the face and fill it with fine water, you will see all the things that are seen from the surface of that ball, in such a way that you will almost see behind your shoulders.