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._