Rhythm in music may be partially described as a peculiar alternating movement, with an alternation between sounds of different pitch, quality, and quantity; between different sound groups, and between sound and silence. The rhythm of visible motion is of a somewhat similar nature, as we shall see in Chapter VIII. But a sense of alternating movement may be produced by things which are not themselves in motion. We can, therefore, find rhythm in fixed lines, shapes, tones, colors, and textures. This we shall call rhythm of fixed design.
The peculiar thing about the element of alternation in rhythm which distinguishes it from mere repetition, is that it is not regular, like the swinging of a pendulum, but contains numerous variations from regularity. But, while the symmetry of rhythm is only partial, so also the variety is limited. It is the combined effect of these two factors which makes rhythm delightful. Repetition or symmetry in a line or a pattern is pleasurable because, as explained in Chapter III, it enables us to see much with ease. But, at the same time, subtle or even bold variations are appealing because they relieve us of monotony, stimulate our interest, and lead our eyes in search of further variations.
A familiar rhythm of line is that of the reverse curve, which Hogarth called "the line of beauty." This line is beautifully used in the painting "Daylight and Lamplight," facing page 39. Observe the effect of alternation with variety in the lines which bound the urn, the woman's figure, and the various shadows and lights in the background. Your eye sweeps over those paths without effort, and you get a sense of movement,
