34 



COMPARATIVE ELECTRO-PHYSIOLOGY 



to become effective by the summation of several. This is 

 seen in fig. 24, where a single vibrational stimulus of 3, 

 alone ineffective, was found to evoke a 

 large response when repeated with rapidity 

 thirty times in succession. 



For the delivering of such equal and 

 rapidly succeeding stimuli, I substitute for 

 the single striker R an eight-spoked wheel, 

 a complete rotation of which, by means of 

 the handle, gives rise to a definite sum- 

 mated effect : and a series of responses to 

 such summated stimulations is found to 

 be uniform. The galvanometer used for 

 these experiments is a dead-beat instru- 

 ment of D'Arsonval type. The sensitive- 

 ness of this is such that a current of io~ 9 

 ampere causes a deflection of I mm. at 

 a distance of I metre. For a quick and 

 accurate method of obtaining records, I 

 devised the following form of response-recorder. The 

 curves are obtained directly, by tracing the excursion of the 



FIG. 24. Additive 

 Effect 



(a) A single stimulus 

 of 3 vibration pro- 

 duced little or no 

 effect, but the same 

 stimulus when 

 rapidly superposed 

 thirty times pro- 

 duced the large 

 effect (b). (Leaf 

 stalk of turnip. ) 



FIG. 25. Response Recorder 



galvanometer spot of light on a revolving drum (fig. 25). 

 This drum, on which is wrapped the paper for receiving 

 the record, is driven by clockwork. Different speeds of 



