How the eye sees spheres and plane surfaces
Studies in vision, with eyes, spheres and visual rays in red chalk
This red-chalk sheet works through how much of an object the eye perceives. Leonardo states that the larger a spherical body is, the less of itself it shows to a motionless eye, and that as such a sphere draws nearer it reveals still less of its surface. Among plane figures, the circle is seen most perfectly because its edges lie equally distant from the eye's centre, while a plane whose ends are unequally distant is judged less perfectly; a sphere seen by one eye also appears smaller than when seen by two. Diagrams of eyes, spheres and visual rays accompany the text, together with a block of arithmetic.
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Larger spheres show less of themselves
The greater the size of a spherical body, the less quantity of itself it shows to the eye, the eye being without motion; and the larger sphere, the closer it approaches the eye, the less of itself it reveals.
The circle is the best-seen plane figure
Among plane surfaces, the one bounded by a circle is seen more perfectly than any other, its edges being equally distant from the centre of the eye. A spherical body seen by a single eye appears smaller, at equal distance, than if seen by two eyes.
Unequal distances judged less well
Of the general class of plane surfaces of even quality, the one whose ends are of more unequal distance from the eye will be judged by it less perfectly.
Column of calculation
A block of numbers and worked operations (4, 31½, 132, 13 4, 144 16, 264 and others) is set at the lower left of the sheet.