HOW THE PAINTER MUST PLACE HIMSELF WITH REFERENCE TO THE LIGHT, TO
GIVE THE EFFECT OF RELIEF.

Let _a b_ be the window, _m_ the point of light. I say that on
whichever side the painter places himself he will be well placed if
only his eye is between the shaded and the illuminated portions of
the object he is drawing; and this place you will find by putting
yourself between the point _m_ and the division between the shadow
and the light on the object to be drawn.