responses to approach
a limit.

[Illustration: FIG. 84.--A SECOND SET OF RECORDS WITH A DIFFERENT
SPECIMEN OF TIN
The amplitudes of vibration are increased by steps of 10 deg., from 20 deg. to
160 deg.. (The deflections are reduced by interposing a high external
resistance.)]

[Illustration: FIG. 85.--EFFECT OF SUPERPOSITION ON TIN
A single stimulus produces the feeble effect shown in the first
response. Superposition of 5, 9, 13 such stimuli produce the
succeeding stronger responses.]

#Threshold of response.#--There is a minimum intensity of stimulus below
which there is hardly any visible response. We may regard this point as
the threshold of response. Though apparently ineffective, the subliminal
stimuli produce some latent effect, which may be demonstrated by their
additive action. The record in fig. 85 shows how individually feeble
stimuli become markedly effective by superposition.

[Illustration: FIG. 86.--INCOMPLETE AND COMPLETE FUSION OF EFFECT IN TIN
As the frequency of stimulation is increased the fusion becomes more and
more complete. Vertical line to the right represents .1 volt.]

#Superposition of stimuli.#--The additive effect of succeeding stimuli
will be seen from the above. The fusion of effect will be incomplete if
the frequency of stimulation be not sufficiently great; but it will tend
to be more complete with higher frequency of stimulation (fig. 86). We
have here a parallel case to the complete and incomplete tetanus of
muscles, under similar conditions.

By the addition of these rapidly succeeding stimuli, a maximum effect is
produced, and further stimulation adds nothing to this. The effect is
balanced by a force of restitution. The response-curve thus rises to
its maximum, after which the deflection is held as it were rigid, so
long as the vibration is kept up.

[Illustration: FIG. 87.--CYCLIC CURVE FOR MAXIMUM EFFECTS SHOWING
HYSTERESIS]

It was found that increasing intensities of single stimuli produced
correspondingly increased responses. The same is true also of groups of
stimuli. The maximum effect produced by superposition of stimuli
increases with the intensity of the constituent stimuli.

#Hysteresis.#--Allusion has already been made to the increased
responsiveness conferred by preliminary stimulation (see p. 127). Being
desirous of finding out in what manner this is brought about, I took a
series of observations for an entire cycle, that is to