The Human Eye: The Pupil's Adaptation to Light and Dark
Nature adjusts the pupil like a window; nocturnal animals and the eye-to-brain ratio
Leonardo describes how the pupil continually changes size with the brightness or darkness before it, narrowing against strong light and widening in darkness, likening Nature's regulation to shutting or opening a window or to the mouth of a purse. He gives an experiment of bringing a candle toward a person's eye to watch the pupil contract, and notes that nocturnal animals such as cats and owls have far larger pupils at night. He adds that the crystalline humour condenses against bright things and rarefies against dark ones, and that nocturnal animals see better at night because their eyes are large relative to their brains, unlike man. This text page carries no diagrams, but marginal headings introduce each section.
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Nature adjusts the pupil like a window
The pupil changes into as many sizes as there are varieties of brightness and darkness before it, narrowing against excess light and widening against darkness like the mouth of a purse. Leonardo compares it to a man who shuts half a window when he has too much light and opens the whole window at night, a continual tempering and adjusting.
A candle experiment on the human pupil
To test the effect on man, watch the pupil while holding a lit candle somewhat apart and have him look at the light, bringing it nearer little by little. The nearer the light approaches, the more the pupil narrows; cats, owls and other nocturnal animals show the reverse, small pupils at midday and very large ones at night.
The crystalline humour in mixed light
When the eye sees bright and dark things at once, the crystalline humour within the pupil condenses opposite the luminous things and rarefies opposite the dark ones. This shows on closing the eye, when the retained images of bright things appear dark and the dark ones appear bright, more so in weak eyes than strong.
Nocturnal animals and the eye-to-brain ratio
Nocturnal animals see better at night because the quantity of their eyes is greater than their whole brain, especially owls, tawny owls, barn owls, little owls and scops owls. Man, by contrast, has a greater brain relative to his eyes than any other land animal, and so sees little after daylight.