A shadow looks darkest against the whitest ground
The angle of contingency on a curved body, and how a bright field deepens the apparent darkness of a shadow
Completing the previous proof, Leonardo shows that a curved body is more lit at its extreme b than at its middle d, because at b it receives both the primitive light a and reflected derivative light e f, while the derivative shadow f g cannot reach past the angle of contingency f b d formed by the straight line f b and the curve b d. Chapter 604 states that, among equally dark objects, the nearer one deepens the opposed shadow more. Chapter 605 argues that among equally dark shadows the one cast on a whiter field looks darkest, proved on a single shadow whose edge appears very dark where it meets white ground and only faintly dark where it borders itself. Two lettered pen diagrams (e, f, g, h, b, d and a, b, d, n, e) illustrate the geometry.
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The angle of contingency on a curved body
A curved body is more illuminated at its extreme b than at its middle d, since at b it sees the primitive light a and, by reflected rays, the derivative light e f. The derivative shadow f g cannot reach it, because f b d is the angle of contingency made by the straight line f b and the curve b d; the rest of the body is seen by f g according as line f g forms the base of a wider or narrower triangle.
The nearer dark object deepens the shadow more (604)
Among objects of equal dark shape and size, that one will augment the darkness of the opposed shadow more which lies nearer to it.
A shadow looks darkest on the whitest field (605)
Among shadows of equal darkness, the one generated on a field of greater whiteness shows itself darkest, and one on a darker field appears less dark. This is proved on a single shadow, whose extreme edge appears very dark where it borders white ground and of little darkness where it borders itself.