Why Rays Bend at the Eye: Vision versus Perspective
Species refract on the eye's surface, so the eye departs from painters' linear perspective
Leonardo argues that the rays (species) coming from objects before the eye do not converge to a single interior point along straight lines: reaching the eye's surface they break, because they pass from a rarer to a denser medium, just as an object seen across the water and air boundary appears out of its place. He contrasts this with painters' linear perspective, showing that the pyramid f c d formed from the painter's point yields a base c d smaller than the base a b formed by the visual rays that break at the eye's surface s t. Labelled eye and perspective diagrams accompany the argument, and further untranscribed sketches of columns and arches appear on the left folio.
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Rays do not meet at one interior point by straight lines
The concourse of the lines created by the species of the objects set before the eye does not converge to a point inside the eye by straight lines.
Species break at the eye's surface, passing rare to dense
If the judgment of the eye is within it, the straight lines of the species break upon its surface, because they pass from the rare to the dense. So too, if you stand under water and look at a thing in the air, you see it out of its place, and likewise the thing in the water seen from the air.
The painters' pyramid f c d versus the visual rays
Painters' perspective at equal distance never shows a thing of the magnitude the eye shows. The pyramid f c d has its point as far from the object c d as f is from the object a b, yet c d (the painter's base) is smaller than a b, the base made by the rays converging to the eye and breaking at s t, the eye's surface; the two are to be tested by cutting the visual and the painters' thread lines on one same plane.