410 COMPARATIVE ELECTRO-PHYSIOLOGY 



negative response, the internal energy may be so increased 

 by the continuous absorption of impinging stimulus as to 

 induce a continuously increasing antagonistic reaction of 

 positivity. Hence, in such cases, negativity is gradually 

 diminished, and ultimately reversed. After the attainment 

 of maximum positivity, however, the negative element once 

 more becomes predominant. 



With a fresh specimen, exhibiting multi-phasic responses 

 under the continuous action of light, I find it easy to obtain 

 a convincing demonstration of the presence of the positive 

 factor by a sudden cessation of stimulus at an appropriate 

 moment. If the stimulus be stopped at ^, the increasing 

 internal energy and natural tendency to recovery will con- 

 spire with each other, and the after-effect will, generally 

 speaking, be b a'. That is to say, this effect is the same as 

 that of natural recovery. 



When the second or positive phase has reached its maxi- 

 mum a', the response under the continued action of stimu- 

 lation is once more reversed to negative a' b\ as we have 

 seen. At this point of reversal a', the positive, is balanced 

 by the negative. If then at this point the impinging stimu- 

 lus be suddenly withdrawn, the positive element, finding 

 itself unopposed, will overshoot the line of balance, and the 

 curve will proceed in the positive direction towards c (fig. 243). 

 This phasic sequence (Type I.) then, during stimulation, and 

 as an after-effect of it, should thus be ( + ), the dotted 

 sign representing the after-effect. That this is actually so, 

 is seen in the following series of photographic records 

 (fig. 244), where the dotted portion of the record exhibits 

 the after-effect The intensity of this positive after-effect 

 varies with the freshness and vigour of the specimen. The 

 influence of fatigue, in the gradual diminution of the effect, 

 is seen in the two subsequent series (b) and (c) obtained 

 from the same specimen. This diminution culminates in 

 the gradual diminution of the positive after-effect, and its 

 conversion into a negative after-effect, as seen in (d). 



We thus arrive at an intermediate case (Type II.), exem- 



