THE ADDITIVE EFFECT 55 



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 repeti- 

 tions. From the record in fig. 22 the effective excitation 



Fig. 23. — Stimulus of intensity - i 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 - i 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 ad- 

 justed 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, in- 

 stead 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 



