The eye, the pupil, and the visual pyramids
How objects diminish and disappear according to their size relative to the pupil
A large geometric diagram sets a semicircle over a field of ruled visual pyramids, with six small numbered circles arranged along the converging rays and a lever-like arm at the left. The accompanying note treats the eye as a circle h K with the pupil f r, and argues that bodies smaller than the pupil are diminished cleanly with distance while a body equal to the pupil cannot produce the same effect except at the side of the eye. Letter and number labels (y c e K r a f h, 1-6, m, n) key the prose to the figure. The page is a study in the optics of vision and perspective diminution.
On this page
The eye as circle h K and the pupil f r
The note supposes the eye to be the circle h K and the pupil to be f r, and reasons that the bodies 1 2 4 5 would then not be seen, nor any that cast a short pyramid behind them. The argument turns on the size of the seen object relative to the pupil.
Diminution of objects with distance
At a certain distance the eye diminishes things without confusion, the further they recede; and from that point on, the nearer the thing comes to the eye the more it is diminished. This holds when the seen thing is smaller than the pupil.
The line y a and the midpoints of the pyramids
The line y a is described as the one that holds within itself all the midpoints of the pyramids. The visual pyramids are laid out geometrically over the semicircle, with the body marked 3 shown to demonstrate the exceptional case at the side of the eye.