OF THE TREATMENT OF THE LIGHTS.
First give a general shadow to the whole of that extended part which is away from the light. Then put in the half shadows and the strong shadows, comparing them with each other and, in the same way give the extended light in half tint, afterwards adding the half lights and the high lights, likewise comparing them together.
Taken from The Notebooks of Leonardo da Vinci edited by Jean Paul Richter, 1880.