Of Lights: Four Kinds of Illumination and Leaf Shadows
How universal, particular, reflected and transmitted light strike opaque bodies
Leonardo opens a discourse 'Of lights', distinguishing four kinds of light that strike opaque bodies: universal (the air within our horizon), particular (sun, window, door), reflected light, and a fourth that passes through translucent things such as cloth or paper. A central semicircular diagram, labelled a b c d e f g K, a o n h and m r S, works out how much of the sky-hemisphere a point on a shadowed body sees, and how the earth darkens the illumination at point S. A closing red-chalk note observes that the shadows in a transparent leaf seen from its reverse are the same as those on its front, though the leaf's sheen can never show through.
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The four kinds of light that illuminate opaque bodies
Light striking opaque bodies is of four sorts: universal, like the air within our horizon; particular, like the sun or a window or door; reflected light; and a fourth that passes through translucent things such as cloth or paper. These translucent bodies act as if nothing were placed between the shadowed body and its illuminant.
How much sky a point on a shadowed body sees (a o n / S)
At point a on the shadowed body n one sees the whole hemisphere b c d e f and none of the earth's darkness, and likewise at o, so the span a o is of one uniform brightness. At S only 4 degrees of the hemisphere d e f g K are seen together with the whole earth S K, which darkens it by as much as calculation gives.
Shadows in a transparent leaf seen from the reverse
The shadows in a transparent leaf viewed from behind are the very same shadows as on its front, which shows through from the reverse together with the luminous part. Only the sheen (lustre) can never show through the leaf.