CLVII
In nature, colour exists no more than line,—there is only light and shade. Give me a piece of charcoal, and I will paint your portrait for you.
Goya.
CLVIII
It requires much more observation and study to arrive at perfection in the shadowing of a picture than in merely drawing the lines of it. The proof of this is, that the lines may be traced upon a veil or a flat glass placed between the eye and the object to be imitated. But that cannot be of any use in shadowing, on account of the infinite gradation of shades, and the blending of them which does not allow of any precise termination; and most frequently they are confused, as will be demonstrated in another place.
Leonardo.