is only one-tenth of what is perceptible to the human subject. Looking at fig. 23 it will be seen that even this very feeble stimulus became effective on being repeated 20 times.
In carrying out this experiment I had expected in a general way that a feeble stimulus, to be effective, must be repeated a greater number of times. But I was not prepared for so strictly quantitative a result as came out in these two records. If the summated effect is to prove strictly additive, then effective excitation must be equal to the individual intensity multiplied by the number of repetitions. From the record in fig. 22 the effective excitation
Fig. 23.—Stimulus of intensity ·1 became effective on being repeated twenty times.
was seen to be ·5 X 4 = 2. From the second record with the same specimen, in fig. 23, it is seen to be ·1 X 20 = 2. In other words, for effective excitation the number of additive stimuli varies inversely as the intensity of each. That this is true, within certain limits, is borne out by another set of results obtained from a different specimen, which was found to be somewhat more excitable than the former.
In order to vary the condition of the experiment I adjusted the reed-interrupter to vibrate twice in a second. There was thus an addition here of the effects of single make-and-break shocks at intervals of half a second, instead of one-fifth of a second as in the last case. In fig. 24 is seen the record of the additive effect, the intensity of stimulus being ·5. We find here that the stimulus became effective on being repeated twice.
The experiment was again repeated with the same
