Light is the chaser away of darkness. Shade is the obstruction of light. Primary light is that which falls on objects and causes light and shade. And derived lights are those portions of a body which are illuminated by the primary light. A primary shadow is that side of a body on which the light cannot fall.
The general distribution of shadow and light is that sum total of the rays thrown off by a shaded or illuminated body passing through the air without any interference and the spot which intercepts and cuts off the distribution of the dark and light rays.
And the eye can best distinguish the forms of objects when it is placed between the shaded and the illuminated parts.
Taken from The Notebooks of Leonardo da Vinci edited by Jean Paul Richter, 1880.