Shadows fade and are lost at long distances because the larger
quantity of illuminated air which lies between the eye and the
object seen tints the shadow with its own colour.
177.
_a b_ will be darker than _c d_ in proportion as _c d_ is broader
than _a b_.
[Footnote: In the original MS. the word _lume_ (light) is written at
the apex of the pyramid.]
178.
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.]