On the Pupil's Adjustment to Light and Dark
Why a bright-dazzled eye sees everything dark, and how the pupil dilates in time
Under the heading 'On perspective', Leonardo explains why an eye passing from sunlit white into a dimmer place finds everything dark: the pupil, contracted by brightness, loses more than three quarters of its visual quantity and hence of its power. He answers the objection about a small bird's tiny pupils by appealing to the proportion of the brain dedicated to sight rather than to pupil size alone, and notes that because the pupil grows and shrinks only over time, the eye cannot adjust instantly between light and dark. He adds that this very effect once deceived him while painting an eye, and that from it he learned the lesson. The full page is text, in mirror script, without diagram.
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The contracted pupil loses visual power
An eye leaving sunlit white for a place of lesser light finds everything dark, because its pupils, contracted at the bright white, shrink to less than a quarter of their former visual quantity and so lose power. The pupil grows and diminishes according to the brightness or darkness of its object, but only over time, so the eye cannot adjust at once between light and dark.
A lesson learned while painting an eye
Leonardo raises and answers the objection of a small bird whose tiny pupils might make white look black, attributing sight to the proportion of the brain devoted to the visual power rather than to pupil size alone. He remarks that this phenomenon of the eye's slow adjustment once deceived him in the act of painting an eye, and that from it he learned the principle.