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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