How a dark object appears thinner against a bright field
Visual-ray pyramids showing dark and bright bodies against fields of differing luminosity
Leonardo argues that a dark object of uniform width looks thinner where it is seen against a brighter background, and conversely that a bright body looks broader against a darker background. He works the case through the pyramid of visual rays, labeling the dark body e and the fields a b and c d (dark, one more than the other) and b c (bright, like sunlight admitted through a hole into a dark chamber). Three pen diagrams of striped bars converging to the eye o accompany the argument.
On this page
Dark bar looks thinner against a bright ground
The object e g appears thicker in the region e f than in g h because e f lies against a darker field; the part f g looks thin because the eye o sees it against the bright field b c, likened to a spot lit by a shaft of sun in a dark room.
Bright body looks broader against a dark ground
Conversely, a luminous body of uniform width and brightness appears thicker where it is seen against a darker field, the effect being taken with the luminous body assumed to be fiery.