DIFFERENT TYPES OF RESPONSE 1 25 



contains a large amount of energy. We may suppose it to 

 consist of a storage battery and a galvanometer, the circuit 

 being normally incomplete. External stimulus may, by some 

 easily arranged mechanism, close the responding circuit 

 periodically. In this case, the responses might be made dis- 

 proportionately larger than the stimulus. There will then be 

 a progressive run-down of the latent energy of the system, 

 and the responses will show diminution, or true fatigue. The 

 system, at the end of the experiment, will be found to be 

 below par. This case may be paralleled by that of highly 

 excitable tissue which is wasted under excessive and long- 

 continued stimulation. 



(3) Stimulus wholly or partially absorbed. — The third type 

 of substance we have supposed to be one which is capable of 

 absorbing and holding latent, to a greater or less extent, the 

 stimulus which it receives, and under this we may have 

 several important sub-cases. 



(a) Staircase and additive effects. — The absorbed stimulus 

 may gradually enhance the molecular mobility, with gradual 

 enhancement of response. This is seen exemplified in the 

 staircase effect. Another example of the same thing is 

 probably to be found in the singly ineffective stimulus which 

 becomes effective on repetition. Evidently in this case, the 

 energy of the first few stimuli is held latent in the tissue and 

 added up until it reaches the threshold of response. 



[b) Multiple response. — We can next see the possibility 

 of a very interesting case of stimulus becoming latent. A 

 spring which is immersed in a viscid fluid may, on receiving 

 a feeble blow, give a single vibrational response. But if the 

 blow be powerful, this single strong stimulus will give rise to 

 a multiple series of vibrational responses. Again, a phos- 

 phorescent substance acted on by light absorbs it, and, on 

 the cessation of incident stimulus, continues to give up the 

 excess of latent energy thus acquired in the form of luminous 

 vibration. A selenium cell again, when acted on by a single 

 strong flash of light, I have found to give what may be 

 regarded as two responses, one strong and the other feeble. 



