And as imagination bodies forth
The forms of things unknown, the poet's pen
Turns them to shapes, and gives to airy nothing
A local habitation and a name.
Shakespeare, Midsummer Night's Dream, V, i.
These then are the problems and the difficulties. The solutions can be only partial and tentative, but they are the best we are able to obtain with our present knowledge and our present capabilities of analysis. As science today has advanced in accuracy of knowledge and understanding of the facts of nature far beyond the powers of our ancestors to imagine, so in the future psychologists may, and let us hope will, enable us to comprehend the subtleties of metrical rhythm beyond our present power. Yet there will always remain, since the ever-inexplicable element of genius is a necessary part of all art, a portion which no science can describe or analyze.
The Psychology of Rhythm. That nearly all persons have a definite sense of rhythm, though sometimes latent because of defective education, is a familiar fact. The origin and source of this sense is a matter of uncertainty and dispute. The regular beating of the heart, the regular alternation of inhaling and exhaling, the regular motions of walking, all these unconscious or semi-conscious activities of the body have been suggested; and they doubtless have a concomitant if not a direct influence on the rhythmic sense. Certainly there is an intimate relation between the heart action and breath rate and the external stimulus of certain rhythmic forces, as is shown by the tendency of the pulse and breath to adapt their tempo to the beat of fast or slow music. But this can hardly be the whole explanation. More important, from the psychological point of view, is doubtless the alternation of effort and fatigue which characterizes our mental as well as physical actions. A period of concentrated attention is at once followed by a period of indifference; the attention flags, wearies, and must be recuperated by a pause, just as the muscular effort of hand or arm. In truth, the muscles of the eye play a real part in the alternations of effort and rest in reading. The immediate application of this psychological fact to the temporal rhythms has been clearly phrased by the French metrist, M. Verrier:
I hear the first beat of a piece of music or of a verse, and, my attention immediately awakened, I await the second. At the end of a certain time—that is, when the expense of energy demanded has reached a certain degree—this second beat strikes my ear. Then I expect to hear the third when the dynamic sense of attention shall indicate an equal expense of energy, that is, at the end of an equal interval of time. Thus, by means of sensation and of memory of the amount of energy expended in the attention each time, I can perceive the equality of time-interval of the rhythmic units. Once this effort of attention becomes definite and fixed, it repeats itself instinctively and mechanically—by reflex action, so to say, like that of walking when we are accustomed to a stride of a given length and rapidity. Here we have truly a sort of metronome which will beat out the rhythm according as we regulate it. And it goes without saying that with this we can not only note the rhythm in our songs or spoken verse or movements, but also perceive it in the sounds and movements of other persons and other things.
This metronome of attention functions, indeed, still more simply. With attention, as with all the psycho-physiological processes, effort alternates with rest: it grows stronger and weaker, contracts and expands in turn. This pulse of attention varies in different persons according to the peculiar rhythm of the organism. In the same person, under normal conditions, it remains nearly constant. It is always subject to modification by the psycho-physiological conditions of the moment, especially by the emotions and by external circumstances. In a series of identical equidistant stresses, those which coincide with the pulse of attention seem the stronger: this is what is called subjective rhythm. Since this coincidence is nearly always somewhat inexact, there results an easy accommodation of the pulse of attention, although even in the subjective rhythm there has already occurred an objective influence capable of affecting us sensibly.[4]
Thus we have always at hand both a more or less efficient bodily metronome in the pulse and in respiration, and also a "cerebral metronome" capable not only of easy adjustment to different rates of speed but also of that subtlest of modulations which psychologists call the 'elastic unit,' and which musicians, though not so definitely or surely, recognize as tempo rubato.
The sense of rhythm, as has been said, differs remarkably in different individuals—just as the sense of touch, of smell, of hearing.[5] To some, rhythm appears chiefly as a series of points of emphasis or stresses alternating with points of less emphasis or of none at all; such are called, in scientific jargon, 'stressers.' To others the principal characteristic of rhythm is the time intervals; such are called 'timers.' But this is a practical, not a philosophical distinction. For it is the succession of points of emphasis which even the most aggressive stresser feels as rhythmic; and succession implies and involves a temporal element. The stresser's only difficulty is to feel the approximate equality of the interval. The essential thing, however, is to understand that, while time is the foundation of speech-rhythm, stress is its universal adjunct and concomitant.[6]
The explanation of this duality is simple. A series of identical tones
