Of the eye.

On the proportions that the sites of the simulacra have which are imprinted upon the eye.

Second.

a c q - o p - r b S

First.

t e S f - a n g c - h

That proportion which the sites of the objects scattered through the fields set before the eye have is never similar to the proportion of the simulacra scattered upon that eye, unless such objects are equidistant from the curvature of the eye. It is proved: let the surface of the eye be a e c, and let the objects scattered through the field set before it be d e S r f. I say that d e S f, being objects equidistant from the surface of the eye, will be sown upon this surface of the eye in the same proportion as they are sown in that field. And this is proved by the ninth of this [book], which says: 'Similar and equal triangles cut by a cut equidistant from their bases will have the cuts in the same proportion among themselves as the bases of such triangles have among themselves.' But if such cuts are not equidistant from their bases, then the cuts will not observe the same proportion as that of these bases. It follows that in the triangle h e r, since the cut n g is equidistant from the base e r, the cut n g is not in the same proportion with e r as is the cut a n (made at the same distance from the angle h) with the base d e of its triangle, which is one half of the base e r.

Doubt concerning the impression of the idol in the eye.

a b c - v x - m - n o - r - p S t q - h

Seek what straightens the species intersected within the pupil.

There is doubt about the situation of the idol in the eye, that is, whether it appears in the concavity of the uvea p, or in the convexity of the crystalline sphere n; but afterwards I am assured how such an idol would not be reflected into the visual power if it were impressed in the concavity of the uvea, since the angle of incidence would not be equal to the angle of reflection; and therefore it may be judged that this idol of c, coming to the light by the line c x, enters the pupil by the line x m, and strikes the crystalline sphere by the line m n, and may rebound at p, the concavity of the uvea, and also pass into the crystalline sphere by the line n r t, and this t be comprehended by the front of the optic nerve l S t, which somewhat penetrates into the crystalline sphere; and thus c on the left sends the simulacrum to t on the left, which it could not do if it were imprinted in the concavity of the uvea at the site p.

Of the species of objects which pass through narrow apertures in a dark place.

a b - o n q - p c d r

It is impossible that the species of bodies which penetrate through apertures into a dark place should not become reversed. It is proved by the third of this [book], which says: 'The particles of every shadowy or luminous ray are always rectilinear.' Therefore the part b of the object a b, passing through the aperture n into the dark place o q p r, will imprint itself on the wall p r at the site c; and the opposite end a of the same object a b will pass through the same aperture n and will imprint itself on the same wall c r at the point r; and thus the right end of such object becomes left and the left becomes right, etc.