The Da Vinci Notebooks
On the choice of light for a picture (549-554).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.