Useful things may be learned by the way, such as Albert Dürer's principle of giving substance to his figures and details, more especially seen in his treatment of drapery, when the lines run into solid black and express the deeper folds and give emphasis and solidity to the figure (p. [183]). The reproductions here given of sketches of drapery by Filippino Lippi and Raphael also show the same principle.
A figure or object of any kind, seen in full light and shade, is relieved at any of its edges either as dark against light, or as light against dark, and we recognize it as a solid form in this way; the boundaries of natural light and shade defining it, and projecting it from the background upon the vision. There may be infinite modulations, of course, between the light part, the half-tones, and the darkest parts; but this broad principle governs all work representing light and shade.
It is, in fact, the principle of the relief of form represented upon a plane surface.
