Which edge of a cast shadow is darkest
Shadow lying between right angles holds the deepest darkness
The folio grades the darkness of a derivative (cast) shadow: the boundary is darkest where it is ringed by the brightest derivative light, and where the shadow falls between differing angles the portion between right angles holds the first degree of darkness. The upper figure, lettered a b c and m n, sets a luminous body against an opaque one to show shadow of varying quality. The lower figure, marked a b, f, c d, e K, m n, treats a cross-shaped opaque body and argues the transverse arm's simple shadow is not carried through to the point of percussion.
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The darkest edge of a cast shadow
That boundary of the derivative shadow is darkest which is surrounded by the brighter derivative light. Contrast with the encircling illumination deepens the edge.
Deepest darkness lies between right angles
Where a derivative shadow is impressed between differing qualities of angles, the part that falls between right angles holds the first degree of darkness. The figure is lettered a b c and m n.
The transverse arm of a cross casts no reaching shadow
If a long luminous body equals the length of a cross-shaped opaque body, the simple derivative shadow of the cross's transverse arm is not carried through to the point of percussion. The construction is marked a b, f, c d, e K, m n.