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,
t