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.
Taken from The Notebooks of Leonardo da Vinci edited by Jean Paul Richter, 1880.