EXPERIMENTS IN STIMULATION, 80 



Luminous Sthnulation. 



It is interesting to note that, in the case of some 

 of the naked-eyed Medusiie, the action of light as 

 a stimulus is most marked and unfailing. In the 

 case of Sarsia, for instance, a flash of light let fall 

 upon a living specimen almost invariably causes it 

 to respond with one or more contractions. If the 

 animal is vigorous and swimming freely in water, 

 the effect of a momentary flash thrown upon it 

 during one of the natural pauses is immediately to 

 originate a bout of swimming. But if the animal is 

 non-vigorous, or if it be removed from the water and 

 spread flat upon an object-glass, it usually gives 

 only one contraction in response to every flash. 

 There can thus be no doubt that a sudden transi- 

 tion from darkness to light acts upon Sarsia as 

 a stimulus, and this even though the transition be 

 but of momentary duration. The question there- 

 fore arises as to whether the stimulus consists in 

 the presence of light, or in the occurrence of the 

 sudden transition from darkness to liMit and from 

 light to darkness. To answer this question, I tried 

 the converse experiment of placing a vigorous 

 specimen in sunlight, waiting till the middle of one 

 of the quiescent stages in the swimming motions 

 had come on, and then suddenly darkening. In no 

 case, however, under these circumstances, did I 

 obtain any response ; so that I cannot doubt it is 

 the light per se, and not the sudden nature of the 

 transition from darkness to light, which in the 

 former experiment acted as the stimulus. Indeed, 



