22 RESPONSE IN THE LIVING AND NON-LIVING 
Photographic recorder.—Or the records may be 
made photographically. A clockwork arrangement 
moves a photographic plate at a known uniform rate, 
and a curve is traced on the plate by the moving spot of 
light. All the records that will be given are accurate 
reproductions of those obtained by one of these two 
methods. Photographic records are reproduced in 
white against a black backeround. 
Compensator.—As the responses are on variation of 
current of injury, and as the current of injury may be 
strong, and throw the 
spot of ight beyond 
the recording surface, 
a potentiometer ba- 
lancing arrangement 
may be used (fig. 9), 
by which the P.D.due 
Laie a ot eens 10 SY ae 
and R’. S isa storage cell. When the key compensated ; E.M. 
K is turned to the right one scale division= 
‘001 volt, when turned to the. left one scale riati ns yrod c d 
division=‘01 volt. P is the plant. va O l uce 

by stimulus are then 
taken in the usual manner. This compensating arrange- 
ment is also helpful, as has been said before, for 
calibrating the E.M. value of the deflection. 
Means of graduating the intensity of stimulus.—One of 
the necessities in connection with quantitative measure- . 
ments is to be certain that the imtensity of successive 
stimuli is (1) constant, or (2) capable of gradual increase 
by known amounts. No two taps given by the hand 
can be made exactly alike. I have therefore devised 
the two following methods of stimulation, which have 
been found to act satisfactorily. 
