Of the eye.

Whether the idol or simulacrum has a determined location upon the eye or not.

a o - a o

The simulacra of motionless objects have determined locations upon motionless eyes. It is proved: let the object be a b, which by the seventh sends the ray from a to a by the shortest way to the pupil a o, and the opposite extreme of the object o does likewise to o. Therefore the simulacrum of the extreme a is not impressed in the location o of the pupil, nor is o impressed in a.

Whether objects send to the eye their simulacra with the parts proportioned as they are in the object.

r m p S - n g q

a - n m - b c d

Objects do not send their simulacra to the eye with the parts proportioned as they are in the object itself. It is proved by the ninth, which says: 'Among things of equal size, the more remote will appear the smaller.'

Let then the eye be a and its object be b c d. I say by the said ninth that d and b, parts of that object, will appear smaller than the part c, because they are more remote from the eye than c, as is proved by the definition of the circle a c. n m is a sector, and b d exceeds the distance that lies from the periphery of the sector to the said eye.

Whether the species of objects are taken by the visual faculty at the surface of the eye, or whether they pass within.

The lenses of spectacles show us how the simulacra of objects settle on the surface of such glasses and from that surface penetrate, bending toward the surface of the eye, from which surface it is possible that the eye sees the figures of the said objects. And this is proved to be possible, because that surface is a common boundary between the air and the eye, for it divides the albugineous humour from the air and divides the air from that albugineous humour. And if we wish to affirm that the species of objects settle on the surfaces of the spectacles, it will be said that in the spectacles of the old the thing shows itself much larger than the real thing; which, were it not for the interposition of such glass between the eye and the object, then that object would show itself of its natural size. Therefore it is a manifest sign that the concourse of species of any object, which shall be cut by the interposition of transparent bodies, will impress itself on the surface of such bodies, and from here regenerates a new concourse, which conducts to the eye the simulacra of such objects.

How the straightnesses of the concourse of species bend on entering the eye.

f - o r t p - c a b d

The straightness of the concourse of the rays bends at the entry they make into the eye, and this is the cause that at equal distance the eye sees the object larger than the rules of the painters' perspective give. And the cause is that the eye receives upon itself the section of the species a b at o p, and the perspectivist, imagining that such rays are straight, cuts the rays at r t, and thus finds the things produced by his perspective much smaller than they are judged by the eye. And if by the adversary it were said that the object a b appeared larger than it is, since the thing that comes to the eye by the lines a o f and b p f appeared in the space c d seen by the straight lines f o c and f p d, this is shown to be impossible, because if you wished to shoot the arrow with the power of the lateral rays f o c, you would think to strike with that arrow at the location c, and it would strike at a.

Do you wish to assure yourself how the rays of luminous bodies are caused by the simulacrum of such bodies, which impress themselves in the thickness of the lids of the eyes, which reflect into the pupil of such eyes? Now open the eye so much that the pupil does not see those lids, and you will see the lights without rays.