What we call "relief "—that is, the apparent standing out of the object from its ground—is effected by the contrasting of light and shadow. So, would you produce breadth and splendour of effect, join together masses of light objects and corresponding masses of dark objects. Again: to get harmony and softness sink some objects wholly or partially in shadow, and let their outlines be insensibly lost in the ground. Finally: to create vivacity and spirit make, in some parts of the picture, abrupt breaks and sharp transitions.
 
Lights and Shadows in a Picture: Their Use and Value [Root's "Camera and Pencil"] reprinted in The British Journal of Photography, April 9, 1875, Vol.XXII, No.779, p.177.