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it is always entirely exposed to one of the lights and to the whole
or part of the other; and it is less deep in proportion as it is
nearer to the two lights, and in proportion as it is turned towards
the outer side x t; because it is more exposed to the second light
a b.
[Footnote: The diagram to this section is given on Pl. V. To the
left is the facsimile of the beginning of the text belonging to it.]
1
84.
OF SIMPLE SHADOWS.
Why, at the intersections a, b of the two compound shadows e f
and m e, is a simple shadow pfoduced as at e h and m g, while
no such simple shadow is produced at the other two intersections c
d made by the very same compound shadows?
ANSWER.
Compound shadow are a mixture of light and shade and simple shadows
are simply darkness. Hence, of the two lights n and o, one falls
on the compound shadow from one side, and the other on the compound
shadow from the other side, but where they intersect no light falls,
as at a b; therefore it is a simple shadow. Where there is a
compound shadow one light or the other falls; and here a difficulty
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