Experiment on the pupil and the apparent size of stars
The eye's aperture responds to the surrounding light; a candle-and-star test
Leonardo sets out an optical experiment on how the pupil enlarges and contracts with the brightness of its surroundings, sketched at the top as two facing heads with a segmented rod and a sight-line running to small circles that stand for luminous bodies. He argues that the darker the sky, the larger the stars appear, and that lighting the intervening air makes them shrink, because the change lies wholly in the pupil. He proposes testing this with a candle held above the head and then lowered toward the line running from a star to the eye, at which point the star nearly vanishes from view.
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Experiment on pupil enlargement and diminution (n–S)
The diagram is captioned as a demonstration of the growth and shrinking of the pupil caused by the motion of the sun or another luminous body. Two heads face a segmented rod, with a line drawn to small circles marking the light sources.
Why stars look larger in a darker sky
The darker the sky, the larger the stars appear; if the intervening medium is illuminated, the same stars look smaller. Leonardo attributes this entirely to the pupil, which widens and narrows with the brightness of the air between the eye and the luminous body.
Candle-and-star demonstration
Hold a candle above the head while gazing at a star, then lower it little by little until it nears the line from the star to the eye; the star will then diminish so much as to be almost lost from sight.