46 COMPARATIVE ELECTRO-PHYSIOLOGY 



galvanometric contacts of equal period at the required 

 intervals. 



The difficulty in this investigation lies in the instan- 

 taneous application of a stimulus at a definite moment, and 

 in the successful adjustment of the subsequent interval at 

 which the resulting responsive current is to be led to the 

 galvanometer and recorded. Instantaneous stimulation can, 

 it is true, be effected by electrical shock. But polarisation, 

 and other disturbances caused by it, might give rise to 

 unknown variations in the responsive effect. Hence, it is 

 advisable when recording the electrical response to employ, 

 if possible, a non-electrical form of stimulus. And it is only 

 after the successful employment of such an unimpeachable 

 method, that we can feel any confidence in the use, after due 

 precautions have been taken, of the electrical stimulus itself, 

 as will be described in a later chapter. Another obstacle to 

 be overcome is the elimination of the unknown element of 

 time required for transmission when stimulus is applied at 

 a distance from the responding point. This uncertainty 

 can only be removed by applying the stimulus directly on 

 the responding point itself. All these difficulties I have 

 successfully met by employing the mechanical form of 

 stimulation, which I am now about to describe. We have 

 seen that a stimulus of definite intensity may be imparted 

 by a quick torsional vibration of either twist or un-twist, or 

 of the one followed by the other. The intensity of this 

 stimulus, as we saw, depends on the angle of torsion, and 

 remains constant as long as that angle is maintained the 

 same. For this purpose I use the vibrational apparatus 

 already described, successive excitations being produced on 

 one side only, say the right. The torsion-head is set by 

 pulling a vertical thread by which the index is made to rest 

 against the stop P. This pull of the vertical thread is 

 against the antagonistic action of the spiral spring s (fig. 36). 

 During the process of the setting, which is carried out slowly, 

 there is a slight excitatory disturbance. But this is allowed 

 to subside. The vertical thread by which the torsion-head 



