Nearer Bodies Have the More Confused Outlines
How the same pupil sees a body's edges in different places, and why closeness blurs them
This page proves that the outlines of an opaque body seen by a single pupil never fall in one and the same place on the body: the lower part b and the upper part a of the pupil each see the edge at different points, projected to different places on the wall behind. It then shows that the nearer a body is to the eye, the more confused its outlines, because the two seen edges are more widely separated. Two diagrams in the text column trace the sight-lines from a pupil a b to bodies e and f against a wall. A closing heading turns to how one should judge which part of a body is more or less luminous.
On this page
One pupil sees a body's edge in two places
For the pupil a b seeing the upper part of the opaque body n: its lower part b sees the body's edge at point d, ending on the wall o r at point e, while its upper part a sees the same edge at point c, ending at f. Since c and d are not in the same place on the body, the intent is proved.
The nearer the body, the more confused its outlines
Outlines of opaque bodies are the more confused the nearer they are to the eye. Pupil a b sees the edges of the near body e at c d widely separated and so confused, while the edges of the more remote body f are nearer together, at n o, and are therefore seen more distinctly.
Two sight-line diagrams of pupil, bodies and wall
Two figures in the text column illustrate the proofs: the upper shows the pupil a, b directed to two circular bodies with a wall o, r and points c, d, e, f; the lower compares the near body e (edges c, d) with the remote body f (edges n, o).