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Chap. CCLXIII.—Of avoiding hard Outlines.
Do not make the boundaries of your figures with any other colour than that of the back-ground, on which they are placed; that is, avoid making dark outlines.
Chap. CCLXIV.—Of Outlines.
The extremities of objects which are at some distance, are not seen so distinctly as if they were nearer. Therefore the painter ought to regulate the strength of his outlines, or extremities, according to the distance.
The boundaries which separate one body from another, are of the nature of mathematical lines, but not of real lines. The end of any colour is only the beginning of another, and it ought not to be called a line, for nothing interposes between them, except the termination of the one against the other, which being nothing in itself, cannot be perceivable; therefore the painter ought not to pronounce it in distant objects.
Chap. CCLXV.—Of Back-grounds.
One of the principal parts of painting is the nature and quality of back-grounds, upon which the extremities of any convex or solid body will always detach and be distinguished in nature, though the colour of such objects, and that of the ground, be exactly the same. This happens, because the convex sides of solid bodies do not receive the light in the same manner with the ground, for such sides or extremities are often lighter or darker than the ground. But if such extremities were to be of the same colour as the ground, and in the same degree of light, they certainly could not be distinguished. Therefore such a choice in painting ought to be avoided by all intelligent and judicious painters; since the intention is to make the objects appear as it were out of the ground. The above case would produce the contrary effect, not only in painting, but also in objects of real relievo.