How the Eye Sees Things Placed Before It
The eyeball, pupil and crystalline humor; images carried to the intellect along the central line a b
A diagram at the upper right shows the ball of the eye as a circle, with its illuminated part cut off by the line S t, and rays converging from points m, n, c, f, g on the left to a single point at the pupil. The long passage below explains how the pupil receives images through the crystalline humor and relays them to the intellect, but only along the central line a b, so that off-axis lines like m n f g are seen yet not considered. Leonardo argues that to count letters set before it the eye must turn from one to another, and that all seen things reach the eye along pyramidal lines whose apex ends in the middle of the pupil.
On this page
Vision perfected only along the central line a b
Things mirrored on the middle of the illuminated surface pass at once into the pupil and are relayed to the intellect, but the pupil presents nothing perfectly except what is directed along the line a b, as the line c a does. The lines m n f g are seen yet not considered, since they are not aligned with a b; to count letters the eye must therefore turn from letter to letter.
The ball of the eye, its light, and the crystalline humor
The ball drawn above is taken as the ball of the eye, and the smaller part cut off by the line S t is its light, the illuminated portion. Images pass into the pupil through a certain crystalline humor that occupies nothing in the pupil which shows itself to the light.
Pyramidal visual lines converging on the pupil
All things seen come to the eye by pyramidal lines, and the apex of that pyramid makes its bound and end in the middle of the pupil, as drawn above. The rays m, n, c, f, g are ruled from the left to this single point.