The Apparent Size of Bright and Dark Objects
How a bright form on a dark ground swells and a dark form on a bright ground shrinks, proved by two experiments
Dense mirror-written notes set out a rule of optics: bright things seen against a dark ground look larger than they are, while dark things against a bright ground look smaller. Leonardo proves the first with a bar of iron heated along part of its length, which seems to swell where it glows, and the second with a rod of uniform width laid half on sunlit paper and half in shadow, so that it appears thin over the light and thick over the dark. Small pen diagrams at the foot of the page, lettered with the eyes a b and the sun h, key the experiment. He attributes the effect to the visual spirits being overwhelmed by an excess of light.
On this page
Bright forms on a dark ground enlarge; dark forms on a bright ground shrink
Bright things in a dark field show larger than they are, and dark things in a bright field diminish. In a bar of iron of uniform thickness with one length heated red, the glowing part looks much thicker than where the iron stays dark; and a cold bar set between the eye and the heated one looks far thinner by comparison.
A rod half on sunlit paper, half in shadow, looks of unequal width
Lay a straight rule of even width on white paper lit by the sun, so that half rests on the lit paper and half beyond it on the shaded floor where the sun does not fall. The part against the sunlit paper appears thin, while the part against the shadow appears thick.
Key to the demonstration: eyes a b, sun h, rod c d
a and b are the eyes; h is the sun striking the floor of a room over the paper n m; e n is the dark floor without paper or sun; c d is the rod, p d the portion lying in sunlight, and n m the portion in shadow.