The Eye's Pupil, and a Water-Glass to Enlarge Moon and Stars
Natural versus artificial perspective of vision, with an instrument to see the moon and stars enlarged
Headed 'Perspective', the page opens with an eye diagram (e, o, p, f and a, b, c, d) and the rule that whatever reaches the eye at a wider angle looks larger. In the 'Main column' Leonardo proposes an artificial optics: because the convex pupil embraces the whole hemisphere and so shows only a single small star, a glass filled with a special water could cut the visual pyramids differently and make the moon appear larger with clearer spots and many small stars each enlarged. Further notes argue that the visual power is not concentrated to a point, since the images of objects are distributed across the pupil just as they are spread through the air and across the starry sky.
On this page
The wider the angle, the larger the object appears
With the eye diagram lettered e o p f and a b c d, Leonardo states the basic rule of apparent size: that thing shows itself larger which reaches the eye under a broader angle.
A glass of water to enlarge the moon and the stars
The convex pupil takes in the whole hemisphere and so reduces the sky to a single star. The art taught in the margin cuts the visual pyramids near the pupil's surface instead, so that many minute stars each show enlarged and the moon appears bigger with its spots more distinct; for this a glass is to be made full of the water described in book 113, Of Natural Things.
Natural perspective and the curvature of the eye
Natural perspective diminishes distant things because the eye's curvature is forced to cut upon itself the pyramids of every image arriving at right spherical angles. The proposed art instead cuts those pyramids at right angles close to the pupil's surface, changing how distant things are reduced.
The visual power is not reduced to a point
Among bodies smaller than the pupil, the nearer one is the less distinct to it, an experiment showing the visual power is not gathered into a single point. The images that meet at the pupil are spread over it just as they are spread through the air, as seen when the whole starry sky appears sown with proportioned stars.