It can be proved why the shadow _o p c h_ is darker in proportion as
it is nearer to the line _p h_ and is lighter in proportion as it is
nearer to the line _o c_. Let the light _a b_, be a window, and let
the dark wall in which this window is, be _b s_, that is, one of the
sides of the wall.

Then we may say that the line _p h_ is darker than any other part of
the space _o p c h_, because this line faces the whole surface in
shadow of [Footnote: In the original the diagram is placed between
lines 27 and 28.] the wall _b s_. The line _o c_ is lighter than the
other part of this space _o p c h_, because this line faces the
luminous space _a b_.

Where the shadow is larger, or smaller, or equal the body which
casts it.

[First of the character of divided lights. [Footnote 14: _lumi
divisi_. The text here breaks off abruptly.]

OF THE COMPOUND SHADOW _F, R, C, H_ CAUSED BY A SINGLE LIGHT.

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.