The shadow _f r c h_ is under such conditions as that where it is
farthest from its inner side it loses depth in proportion. To prove
this:
Let _d a_, be the light and _f n_ the solid body, and let _a e_ be
one of the side walls of the window that is _d a_. Then I
say--according to the 2nd [proposition]: that the surface of any
body is affected by the tone of the objects surrounding it,--that
the side _r c_, which faces the dark wall _a e_ must participate of
its darkness and, in the same way that the outer surface which faces
the light _d a_ participates of the light; thus we get the outlines
of the extremes on each side of the centre included between them.]
This is divided into four parts. The first the extremes, which
include the compound shadow, secondly the compound shadow between
these extremes.
179.