Every body in light and shade is situated between 2 pyramids one
dark and the other luminous, one is visible the other is not. But
this only happens when the light enters by a window. Supposing _a b_
to be the window and _r_ the body in light and shade, the light to
the right hand _z_ will pass the object to the left and go on to
_p_; the light to the left at _k_ will pass to the right of the
object at _i_ and go on to _m_ and the two lines will intersect at
_c_ and form a pyramid. Then again _a_ _b_ falls on the shaded body
at _i_ _g_ and forms a pyramid _f_ _i_ _g_. _f_ will be dark because
the light _a_ _b_ can never fall there; _i_ _g_ _c_ will be
illuminated because the light falls upon it.

Light and shadow with regard to the position of the eye (141--145).