Chap. CLXXXVIII.—Advice to Painters.
Be very careful, in painting, to observe, that between the shadows there are other shadows, almost imperceptible, both for darkness and shape; and this is proved by the third proposition [38], which says, that the surfaces of globular or convex bodies have as great a variety of lights and shadows as the bodies that surround them have.
Chap. CLXXXIX.—Of Shadows.
Those shadows which in Nature are undetermined, and the extremities of which can hardly be perceived, are to be copied in your painting in the same manner, never to be precisely finished, but left confused and blended. This apparent neglect will shew great judgment, and be the ingenious result of your observation of Nature.
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.
Chap. CXCI.—Whether the Light should be admitted in Front or sideways; and which is most pleasing and graceful.
The light admitted in front of heads situated opposite to side walls that are dark, will cause them to have great relievo, particularly if the light be placed high; and the reason is, that the most prominent parts of those faces are illumined by the general light striking them in front, which light produces very faint shadows on the part where it strikes; but as it turns towards the sides, it begins to participate of the dark shadows of the room, which grow darker in proportion as it sinks into them. Besides, when the light comes from on high, it does not strike on every part of the face alike, but one part produces great shadows upon another; as the eyebrows, which deprive the whole sockets of the eyes of light. The nose keeps it off from great part of the mouth, and the chin from the neck, and such other parts. This, by concentrating the light upon the most projecting parts, produces a very great relief.