A Treatise on Painting/Chapter 190
Chap. CXC.—Of the Kind of Light proper for drawing from Relievos, or from Nature.
Lights separated from the shadows with too much precision, have a very bad effect. In order, therefore, to avoid this inconvenience, if the object be in the open country, you need not let your figures be illumined by the sun; but may suppose some transparent clouds interposed, so that the sun not being visible, the termination of the shadows will be also imperceptible and soft.