EXPERIMENTS IN STIMULATION, 39) 
Luminous Stumulution. 
It is interesting to note that, in the case of some 
of the naked-eyed Medusz, 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 light 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, 
