584 COMPARATIVE ELECTRO-PHYSIOLOGY 



The critical point of reversal would in this instance appear 

 to be slightly below i volt, in the case of the kathode, while 

 in that of the anode, it was at I volt, or slightly above. The 

 effect observed at the extreme points were the same in all 

 cases. Individual differences were concerned only with the 

 exact point of reversal. Thus the point of reversal for the 

 kathode varied in different cases between -6 and I volt ; 

 whereas with the anode it varied from I to 1*5 volt. In a 

 subsequent chapter,, this phenomenon of reversal of sensation 

 under varying intensities of E.M.F., when other forms ot 

 stimulus are applied, will be studied in more detail. It may 

 be stated here, however, that though the critical point of 

 reversal varies to some extent with different individuals, and 

 under different forms of stimulation, yet the law holds good 

 that the excitatory effects induced by moderate E.M.F. are 

 exactly reversed under feeble. 



The main results regarding this opposition of the effects 

 of feeble and strong E.M.F. may be still better demonstrated 

 by the method of successive contrasts. In the last experi- 

 ments, a long course of observations on the same individual, 

 would be liable to fatigue the tissue. Moreover, the fine 

 gradation of the changes induced is not calculated to exhibit 

 the contrasts involved in their full intensity. Having, then, 

 determined, from the previous experiments, that an E.M.F. 

 of '5 volt and another of 2 volts were opposed in their 

 excitatory effects, I now made special arrangements for 

 applying these two intensities of E.M.F. alternately. For 

 this purpose, I arranged a potentiometer which gave an 

 E.M.F. of 2 volts between L and N (fig. 359), and ot 

 5 volt between L and M. The end, L, was connected with 

 the wound-spot by means of a non-polarisable electrode. 

 A distant indifferent point, say on the surface of the finger, 

 was connected with a double key K K'. When K' was 

 pressed, -5 volt was applied, and when K, 2 volts. Further, 

 by means of a reversing-key, P, the wound-spot could be 

 made either anode or kathode at will. In this way, first 

 making the wound-spot anode, I applied alternately the 



