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Let the light on a flat surface be a, and the object which forms the
primary shadow be bc. The wall de is the surface on which the
derivative shadow is received at the place mn. The remainder,
dn and me, continues to be illuminated by a, and the
light from dn is reflected in the primitive shadow bc, and
the light from me does the same. Thus the derivative shadow
nm, which is not exposed to the light a, remains dark, while
the primitive shadow is illuminated by the lit background which surrounds
the derivative shadow. Therefore the derivative is darker than the
primitive.
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