w in the last chapter that increase of strain necessitates a longer
period of recovery. Thus the longer a wire is stimulated, the more and
more overstrained it becomes, and it therefore requires a gradual
prolongation of the interval between the successive stimuli, if recovery
is to be complete. This interval, however, being maintained constant,
the recovery periods virtually undergo a gradual reduction, and
successive recoveries become more and more incomplete. These
considerations may be found to afford an insight into the progressive
diminution of response in fatigued substances.

[Illustration: FIG. 72.--DIMINUTION OF RESPONSE DUE TO SHORTENING THE
PERIOD OF RECOVERY
The stimulus is maintained constant. In (_a_) the interval between two
successive stimuli is one minute, in (_b_) it is half a minute, and
in (_c_) it is again one minute. The response in (_b_) is feebler
than in either (_a_) or (_c_).]

#Fatigue under continuous stimulation.#--Fatigue is perhaps best shown
under continuous stimulation. For example, in muscles, when fresh and
not fatigued, the top of the tetanic curve is horizontal, or may even be
ascending, but with long-continued stimulation the curve declines. The
rapidity of this decline depends on the nature of the muscle and its
previous condition.

In metals I have found exactly parallel instances. In tin, so little
liable to fatigue, the top of the curve is horizontal or ascending; or
it may exhibit a slight decline. But the record with platinum shows the
rapid decline due to fatigue (fig. 73).

[Illustration: FIG. 73
(_a_) The top of response-curve under continuous stimulation in tin
is horizontal or ascending as in figure.
(_b_) In platinum there is rapid decline owing to fatigue.]

Taking any of these instances, say that in which fatigue is most
prominent, it is found that short period of rest restores the original
intensity of response. This affords additional proof of the fact that
fatigue is due to overstrain, and that this strain, with its sign of
attendant fatigue, disappears with time.

#Staircase effect.#--We shall now discuss an effect which appears to be
the direct opposite of fatigue. This is the curious phenomenon known to
physiologists as 'the staircase' effect, in which successive uniform
stimuli produce a series of increasing responses. This is seen under
particular conditions in the response of certain muscles (fig. 74, _a_).
It is also observed so