466 Journal of Comparative Neurology and Psychology. 



II the black box was much darker than it was for those of set I, 

 whereas the white box was not markedly different in appearance. 



Special conditions of set III. The experiments of this set were 

 conducted with the visual conditions the same as in set II, except 

 that there was no hole in the white cardboard over the electric 

 box. This rendered the white box much darker than it was in the 

 experiments of set II, consequently the two boxes differed less 

 in brightness than in the case of set II, and discrimination was 

 much more difficult than in the experiments of either of the other 

 sets. 



In the second column of table 2 the values of the several 

 strengths of electrical stimuli used in the investigation are stated. 

 To obtain our stimulus we used a storage cell, in connection with 

 gravity batteries, and with the current from this operated a Porter 

 inductorium. The induced current from the secondary coil o- 

 this apparatus was carried by the wires which constituted an inter- 

 rupted circuit on the floor of the electric boxes. For the experi- 

 ments of set I the strengths of the stimuli used were not accurately 

 determined, for we had not at that time discovered a satisfactory 

 means of measuring the induced current. These experiments 

 therefore served as a preliminary investigation whose chief value 

 lay in the suggestions which it furnished for the planning of later 

 experiments. The experiments of sets II and III were made 

 with a Porter inductorium which we had calibrated, with the 

 help of Dr. E. G. Martin of the Harvard Medical School, by a 

 method which he has recently devised and described.^ 



On the basis of the calibration measurements which we made 

 by Martin's method the curve of fig. 3 was plotted. From this 

 curve it is possible to read directly in "units of stimulation" the 

 value of the induced current which is yielded by a primary cur- 

 rent of one ampere for any given position of the secondary coil. 

 With the secondary coil at o, for example, the value of the induced 

 current is 350 units; with the secondary at 5.2 centimeters on the 

 scale of the inductorium, its value is 155 units; and with the second- 

 ary at 10, its value is 12 units. The value of the induced current 

 for a primary current greater or less than unity is obtained by 

 multiplying the reading from the calibration curve by the value 



^ Martin, E. G. A quantitative studv of faradic stimulation. I. The variable factors involved. 

 Amer. Jour, oj Physiol. ,\o\. 21, Yi^.(i\--]\. 1908. II. The calibration of the inductorium for break 

 shocks. Ihid., pp. 1 16-132. 



