VARIOUS TYPES OF RESPONSE 65 



in the record will be due to changes of excitability induced 

 as an after-effect of the stimulus itself. 



Maintaining the stimulus-intensity absolutely constant 

 is not so easy to secure with a single make- or break-shock, 

 since the intensity of such a shock is liable to variation, 

 according to the degree of suddenness with which it is 

 effected ; but the total additive value of a group of such 

 shocks may be expected to be fairly constant. For this 

 reason, therefore, tetanising shocks caused by a vibrating 

 interrupter would be preferable, provided the duration of 

 these shocks, depending on the duration of closure of 

 current in the primary circuit, be maintained in successive 

 experiments rigorously equal. Such constancy cannot be 

 arrived at if the closure of the circuit be caried out by hand, 

 or even by metronome. Some special mechanical device 

 must therefore be adopted for this purpose. 



It is further necessary, in order to maintain constancy of 

 conditions, that identical periods of recovery should be 

 allowed in successive records. For this the stimulus must 

 be applied at accurate and pre-determined intervals of 

 time. The ideal condition, then, for the final elimination 

 of all uncertainties due to the personal factor, is that the 

 plant attached to the recording apparatus should be auto- 

 matically excited by a stimulus absolutely constant, make 

 its own responsive records, go through its own period of 

 recovery, and embark on the same cycle over again without 

 assistance at any point on the part of the observer. 



These demands have been fully met in the devices and 

 adjustments now to be described, consisting as they do 

 of two chief elements — namely, the Periodic Starter and 

 the Automatic Exciter. By the former the time-interval 

 between successive stimulations is regulated ; by the latter 

 the stimulation itself, of a definite duration, is applied. 



The Periodic Starter 



In the case of Mimosa recovery from excitation is 

 practically completed in a period of about 10 to 20 minutes, 



