HOW AND WHEN THE SURROUNDINGS IN SHADOW MINGLE THEIR DERIVED SHADOW
WITH THE LIGHT DERIVED FROM THE LUMINOUS BODY.

The derived shadow of the dark walls on each side of the bright
light of the window are what mingle their various degrees of shade
with the light derived from the window; and these various depths of
shade modify every portion of the light, except where it is
strongest, at _c_. To prove this let _d a_ be the primary shadow
which is turned towards the point _e_, and darkens it by its derived
shadow; as may be seen by the triangle _a e d_, in which the
angle _e_ faces the darkened base _d a e_; the point _v_ faces the
dark shadow _a s_ which is part of _a d_, and as the whole is
greater than a part, _e_ which faces the whole base [of the
triangle], will be in deeper shadow than _v_ which only faces part
of it. In consequence of the conclusion [shown] in the above
diagram, _t_ will be less darkened than _v_, because the base of the
_t_ is part of the base of the _v_; and in the same way it follows
that _p_ is less in shadow than _t_, because the base of the _p_ is
part of the base of the _t_. And _c_ is the terminal point of the
derived shadow and the chief beginning of the highest light.

[Footnote: The diagram on Pl. IV, No. 5 belongs to this passage; but
it must be noted that the text explains only the figure on the
right-hand side.]