Of the eye.

Why the right-hand thing does not appear on the left in the eye.

a - r b c e - f g h

S - t - r - h n m o p

The species of objects, in their entry made into the eye, bend the straightness of their rays in the manner proved in perspective, when these species pass from the density of the water to the rarity of the air. But to return to the subject of the right-hand thing that does not appear on the left in the eye, we see a manifest experience: the species that penetrate into the albugineous humour through the pupil of the eye strike upon the sphere of the crystalline humour, in which two things are to be considered, namely either that the visual power is in it, or that it is at the front of the optic nerve, which front takes these species and refers them to the common sense, as do those nerves of smell. And if this power is at the centre of the crystalline humour, it takes the species with its surface: either they are referred to it from the surface of the light [of the eye], where the objects are mirrored, or they are reflected to it from the surface of the uvea, which is the boundary and covering of the albugineous humour, which has darkness behind the transparency of the albugineous humour, just as behind the transparency of glass is placed the darkness of the lead, so that things may better mirror themselves in the surface of such glass. But if the visual power is at the centre of the crystalline sphere, then all the things given to it from the surface of the light of the eye will appear to it in the true place where they are, and will not be exchanged from right to left, and they will appear larger, as is proved in perspective. And if this crystalline sphere takes such species reflected from the concavity of the uvea, it will take them straight, even though the uvea is a concave mirror; and it will take it straight, because the centre of the crystalline sphere is concentric with the centre of the sphere of the uvea. It is true that the species which pass to this uvea, which are outside the eye, pass to this uvea through the centre of the crystalline sphere and, having reached the uvea, become reversed; and the same do those which pass to this uvea without passing through such humour. Therefore we will try to make this visual power be at the front of the optic nerve. We will say that from there one sees in the crystalline sphere all the things taken by it to be set straight, because those which were made reversed in the uvea, it takes and reverses them once again, and thus this crystalline sphere presents straight to the optic [nerve] the species which were given to it reversed. It will perhaps be said that the spherical surface of the crystalline, united with the sphere of the albugineous, does not change its nature, and it is as if the whole were albugineous, and for this reason the albugineous sphere would not have that office which it would have were it surrounded by air. But here one answers that such an effect cannot happen, because the ball of crystal placed in water performs the same office as in the air.

Break a glass flask, and out of its convexity and concavity make yourself a mask full of water, and you will see that what is promised below is true. - And if you wish to see with a single eye, make it with the body of a small or large phial, etc.

a - lukewarm water - n p

o - water

b a - r t - a b; r t will be large at the pupil.

Here the air becomes a convex mirror.

Make little spherical phials, thus: (figure of a phial), and then cut them as one cuts screw-glasses with the hot iron, and make of them shells of half a sphere, thus: (double figure), and then make your spectacles full of water, thus: (figure), and fill only one full of water.