The human eye.

How things on the right will not appear on the right to the visual faculty, unless their species pass through two intersections.

a b - c d - e f - L n v - o - p q r - S - t i g h

The object K, having reached b, the surface of the light of the eye, gives itself to the visual faculty through the 2 intersections, that is n and S, entering first at b into d, the pupil, and from d to f. And it would pass through the intersection n, the centre of the sphere of the light, that is L a b v, but it meets first the crystalline sphere, before its pyramid c d n ends; in that impact the pyramid c d n is cut at e f, where the base of another pyramid is generated, that is e f o, and the sides of that pyramid intersect at o and pass to the opposite part of the vitreous sphere, and the right f becomes left at q, and the left e becomes right at r. Then the second intersection is made at S, that is, the vitreous sphere casts its pyramid q r S, and the right r passes through the intersection S and becomes left at i, and the left q becomes right at g; and by this way the eye, as instrument, presents to the front of one of the optic nerves the thing on the right as on the right and the left as on the left.

Second opinion, with the same 2 intersections.

n - a c d b - e - r t - f - h i K - r - g q L

It is not denied that all the species that come to the surface of the light a n b pass through the pupil c d, as is proved; and it is further granted that all the lines of those species that pass the centre of the sphere which begins at the said light exchange from right to left and from left to right, that is, that a on the right, having passed the centre of the sphere of the light, becomes left at t, and b becomes right at r and strongly grows, because the whole space a b comes to r t, and that r t receives a size that appears similar to a b, as is proved in perspective. Then it is not denied that the vitreous sphere r t K h renders straight within the pyramid K h r, and then reverses it after the intersection of the sides of that pyramid at the points g L. Therefore a on the left will become right at t after the first intersection e, and will descend left as far as K, and will pass through the second intersection r, and will return left at the point g.

Why things diminished by the point of perspective appear much smaller than the natural ones, even though they be measured to be of the same size as the said diminished ones.

a b large refers itself to r t small, and of the same smallness penetrates into K h and ends at g L, still diminished; and it does in the likeness of the concave mirror a b c d, before which, the object e n being set, that object is seen mirrored, having grown by the whole space b c; and the object f p does likewise, but e n has its simulacrum upright in the mirror, and f p has its simulacrum upside down, and such simulacra will be of equal size, their causes being equal. But before I confirm this proposition, I will make experiment of it.

a b c d - e f - o - n p

With this proof one will come to the knowledge of spectacles, because, being interposed between the eye and the object, they make that object equal, or much larger, or smaller.