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The motion of a shadow is always more rapid than that of the body
which produces it if the light is stationary. To prove this let a
be the luminous body, and b the body casting the shadow, and d
the shadow. Then I say that in the time while the solid body moves
from b to c, the shadow d will move to e; and this
proportion in the rapidity of the movements made in the same space
of time, is equal to that in the length of the space moved over.
Thus, given the proportion of the space moved over by the body b
to c, to that moved over by the shadow d to e, the proportion
in the rapidity of their movements will be the same.
But if the luminous body is also in movement with a velocity equal
to that of the solid body, then the shadow and the body that casts
it will move with equal speed. And if the luminous body moves more
rapidly than the solid body, the motion of the shadow will be slower
than that of the body casting it.
But if the luminous body moves more slowly than the solid body, then
the shadow will move more rapidly than that body.
SIXTH BOOK ON LIGHT AND SHADE.
The effect of rays passing through holes (213. 214).
2
13.
167
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