THE INFLUENCE OF SECONDARY RESISTANCE 8l 
a means of expressing the specific value of any break in- 
duction shock, no matter how the factors concerned in 
its production may vary, we must recognize that in the 
ordinary practice of the physiologist the attempt to 
make use of this formula presents very considerable 
difficulties. These difficulties, moreover, are chiefly in 
connection with the inclusion of the factors R and A, 
and we may well inquire how great errors are likely to 
arise in comparing faradic stimuli if these two factors 
are completely disregarded. 
We must realize at the outset of this part of our in- 
quiry that if comparisons are attempted between stimuli 
used under conditions of widely varying secondary re- 
sistance and divergent cathode surface, disregard of 
these two factors is sure to lead to erroneous conclu- 
sions; but probably in a majority of physiological 
experiments the stimuli to be compared are produced 
under conditions which tend to be closely similar. With 
regard to such cases as these we may properly inquire 
whether the factors under consideration need be taken 
into account. 
Successive Stimulation of the Same Tissue. Prob- 
ably the experiments in which accurate comparisons of 
stimuli are most needed are those in which a given 
tissue is to be stimulated successively. But in experi- 
ments of this class neither the tissue resistance nor the 
electrode surfaces undergo noteworthy variation during 
