How the Arch of the Sky Illuminates Bodies Unequally
Sites e and f lit by greater and lesser sky; fields matched to bodies; curved and faceted forms
Continuing on universal light, Leonardo proves with a marginal diagram that the sky illuminates bodies unequally. The arch of the sky a b c d lights the bodies beneath it, but the recessed site f, seen only from the small sky-portion c d, receives far less light than the site e, which is open to the portion a b, three times greater — so e is three times brighter than f. He then turns to fields proportioned to their bodies, noting a flat surface of uniform colour and light will not separate from a like field, and begins classing regular bodies into the curved (oval or spherical) and the faceted, whether regular or irregular.
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The arch of the sky a b c d and unequal light at e and f
The marginal figure sets a b c d for the arch of the sky, universal illuminator of the bodies beneath it, with n m the bodies bounding the space s t r h between them. The site f is lit only by the small sky-portion c d, whereas the site e sees the portion a b, three times greater; therefore e is three times more illuminated than f.
Fields matched to the bodies set against them
Of flat surfaces of uniform colour, a field of the same colour and light will not appear separated from the surface set against it; conversely, differing fields will appear to separate. Leonardo then begins dividing regular bodies into two sorts: those clothed in a curved, oval or spherical surface, and those bounded by faceted surfaces, regular or irregular.