How the pupil adjusts and a light keeps its size at distance
Why the eye perceives a receding light as dimmer but not smaller
This optics folio argues that a light source seen from farther away loses brightness but not apparent size, the eye registering only a diminished splendour. Leonardo explains that nature makes the pupils of all animals, of water and of land alike, contract like a purse in strong light and dilate in darkness. As the pupil widens, a faint light and the bodies it reveals seem to grow. The passage closes with a cross-reference to sheet 37.
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The pupil contracts in strong light and dilates in darkness
Nature has set the pupils of all animals, of water and land alike, to enlarge or contract with the light: in strong light the pupil draws tight like a purse, and in darkness it opens wide. As it opens, the diminished light seems to grow, and with it the apparent size of the bodies seen by that light.
Apparent size versus brightness of a distant light
A thing seen by a greater light of the eye appears larger, and smaller when the contrary holds. A light placed between us and an object does not shrink with distance except in brightness of splendour, and not in its size.