| 149 | 150 | 151 | 152 | 153 |
| 1 | 306 | 613 | 919 | 1225 |
|
which causes the shadow s a, and the shadow x a.
But if you uncover both the lights a b, then you get the two
shadows n m both at once, and besides these, two other, simple
shadows are produced at r o where neither of the two lights falls
at all. The grades of depth in compound shadows are fewer in
proportion as the lights falling on, and crossing them are less
numerous.
1
86.
Why the intersections at n being composed of two compound derived
shadows, forms a compound shadow and not a simple one, as happens
with other intersections of compound shadows. This occurs, according
to the 2nd [diagram] of this [prop.] which says:--The intersection
of derived shadows when produced by the intersection of columnar
shadows caused by a single light does not produce a simple shadow.
And this is the corollary of the 1st [prop.] which says:--The
intersection of simple derived shadows never results in a deeper
shadow, because the deepest shadows all added together cannot be
darker than one by itself. Since, if many deepest shadows increased
in depth by their duplication, they could not be called the
deepest shadows, but only part-shadows. But if such intersections
are illuminated by a second light placed between the eye and the
intersecting bodies, then those shadows would become compound
shadows and be uniformly dark just as much at the intersection as
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