Why a sphere seen with two eyes never looks perfectly round
Binocular vision and the apparent shape of an opaque sphere
Leonardo states that no opaque spherical body seen by both eyes will ever appear of perfect roundness. He marks the position of the right eye a and the left eye b, and explains that closing one eye changes the centre around which the sphere is seen. A small diagram of two intersecting circles beside the text stands for the overlapping fields of the two eyes.
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A sphere never appears perfectly round to two eyes
Leonardo asserts that any opaque body of spherical shape, when looked at with both eyes at once, can never present itself as perfectly round. The claim opens a study of how binocular vision distorts the apparent outline of a solid.
Right and left eye as points a and b
The point a marks the site of the right eye and b the site of the left. Closing the right eye makes the spherical body appear about one centre; closing the left eye shifts the body so that it surrounds the other centre. The two viewpoints thus place the sphere differently.
Two intersecting circles at a and b
A pair of overlapping circles is drawn to the right of the text, their shared region standing for where the two eyes' views coincide. The circles carry the labels a and b for the eye positions.