EXPERIMENTS IN STIMULATION. CI 



persons susceptible to this kind of stimulation, the 

 muscular action passes completely beyond the power 

 of the will. Lastly, I may further observe, what 

 I do not think has ever been observed before, that 

 even in the domain of psychology the action of this 

 principle admits of being clearly traced. We find 

 it, for instance, in the rhythmical waves of emotion 

 characteristic of grief, and at the other extreme we 

 find it in the case of the ludicrous. We can endure 

 for a short time, without giving any visible re- 

 sponse, the psychological stimulation which is 

 supplied by a comical spectacle; but if the latter 

 continues sufficiently long in a sufficiently ludicrous 

 manner, our appropriate emotion rapidly runs up 

 to a point at which it becomes uncontrollable, and 

 we burst into an explosion of ill-timed laughter. 

 But in this case of psychological tickling, as in the 

 previous case of physiological tickling, some persons 

 are much more susceptible than others. ^Neverthe- 

 less, there can be no doubt that from the excitable 

 tissues of a plant, through those of a jelly-fish and 

 a frog, up even to the most complex of our psj^cho- 

 logical processes, we have in this recently discovered 

 principle of the summation of stimuli a very 

 remarkable uniformity of occurrence. 



Effects of Temjperatiire on Excitability. 



I shall now conclude this chapter with a brief 

 statement of the effects of temperature on the 

 excitability of the Medusae ; and before stating my 

 results, I may observe that in all my experiments 

 in this connection I changed the temperature of the 



