Pupil Size, Visual Angle and the Rays of Bright Bodies
The glowing iron rod, and why one light may make two or three images in a single eye
Continuing the optics of the eye, Leonardo states that a smaller pupil sees an object smaller and darker, and that among equidistant bodies the smaller subtends a smaller visual angle (a b d less than b c d). He notes that a brighter body seems larger, illustrated by a red-hot iron rod that looks thicker along its glowing length though its true thickness is uniform. A final passage explains how one luminous body can produce two or three images in a single eye when it is narrowed, with rays rebounding from the eyelids.
On this page
A smaller pupil sees smaller and darker
In an eye of a given size, the smaller pupil sees the object both smaller and darker — Leonardo's opening proposition on this page.
Visual angle and the glowing iron rod
Among equidistant bodies the smaller sends a smaller angle to the eye (angle a b d less than b c d), and a body that loses more splendour appears smaller. A red-hot iron rod of uniform thickness looks much thicker along its glowing part in a dark place, the more so the hotter it is, because a brighter body casts longer visible rays.
One light making two or three images in the eye
When the eye is narrowed against strong light and the head inclined, a single light can form two images (figure a) or three (figure b), as rays rebound from the moist upper and lower eyelids to the pupil.