Perspective, refraction, and the visual pyramid of the eye
Why painters' perspective understates true size; rays breaking on the eye's surface
Leonardo argues that the rays carrying the images (species) of objects do not converge to a single point inside the eye along straight lines, but break upon its surface where they pass from a rarer to a denser medium, just as an object seen across the water-air boundary appears displaced from its true place. He then contrasts the painters' construction with true vision, showing that for a pyramid f c d the base c d fixed by the painters' point comes out smaller than the base a b formed by the visual rays that break at the eye's surface s t, and proposes to test both by cutting the lines on a single plane. The sheet is crowded with further untranscribed drawings, including regular polyhedra and polygons inscribed in circles.
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Rays do not meet at one point within the eye
The concourse of the lines created by the species of the objects set before the eye do not converge to a point inside the eye along straight lines. If the judgement of the eye is within it, the straight lines break on its surface because they pass from the rare to the dense.
An object seen across the water-air boundary is displaced
If you stand under water and look at a thing in the air, you will see it out of its true place, and likewise for a thing in the water seen from the air. Leonardo uses this refraction to argue that the visual rays bend at the surface of the eye.
Painters' perspective understates the eye's magnitude
For the pyramid f c d the apex is as far from object c d as point f is from object a b, yet the base c d made by the painters' point is smaller than base a b made by the visual rays converging to the eye and breaking at the surface s t. He proposes to test this with visual lines and with the painters' thread-lines on one plane.
Regular solids and polygons inscribed in circles
The lower right of the sheet carries a group of drawn geometric solids and many-sided polygons set inside circles. These figures are not covered by the transcription and are noted here only from what is drawn.