Suppose, now, it is our desire to compare the results of reactions to different intensities of electrical and tactual stimuli; let the figures be as follows: 

Reaction Time.            Variability. 
Stimulus Strength.   Elect.       Touch.       Elect.     Touch.
8            50[sigma] 50[sigma]    10[sigma]  10[sigma].
4           130        155          25         30
2           175        220          40         40
1           300        320          50         60

In the double columns the results for electrical stimuli are given first, and in the second column are the tactual.  Stimulus 8 is assumed to be of sufficient strength to induce what may be designated as forced movement, and whatever the quality of the stimulus this reaction time is constant.  I make this statement theoretically, although all the evidence which this work furnishes is in support of it.  So, likewise, is the variability of this type of reaction time small and nearly constant.  At the other extreme, stimulus 1 is so weak as to be just sufficient to call forth a response; it is the so-called threshold stimulus.  Whether all qualities of stimulus will give the same result here is a question to be settled by experimentation.  Wundt contends that such is the case, but the observations I have made on the electrical and tactual reactions of the frog cause me to doubt this assumption.  It seems probable that the ’just perceptible stimulus reaction time’ is by no means the same thing for different qualities of stimulus.  Those modifications of the vital processes which alone enable organisms to survive, make their appearance even in the response to the minimal stimulus.  In one case the just perceptible stimulus may cause nothing more than slight local changes in circulation, excretion, muscular action; in another it may produce, just because of the particular significance of the stimulus to the life of the organism, a violent and sudden motor reaction.  But grant, if you will, that the threshold reaction time is the same for all kinds of stimuli, and suppose that the variability is fairly constant, then, between the two extremes of stimuli, there are gradations in strength which give reaction times of widely differing variabilities.  If, now, at some point in the series, as, for instance, to stimulus 2, the variability for different kinds of stimuli is the same either with reference to the reaction time (ratio) or absolutely, what interpretation is to be put upon the fact?  Is it to be regarded as merely a matter of chance, and unworthy of any special attention, or should it be studied with a view to finding out precisely what variability itself signifies?  It is obvious that any discussion of this subject, even of the possible or probable value of variability as a criterion for the comparative study of stimuli, can be of little value so long as we do not know what are the determining factors of variations of this sort.  The only suggestion as to the meaning of such a condition (i.e.,