Sensory Discrimination: Vision 143 



slowness, as compared with similar reactions to other 

 stimuli, or its dependence on a special organ or region 

 of the body, give evidence that it is accompanied by a 

 specific sensation quality. 



39. The Continuous Action of Light: Photokinesis 



When light stimulates not through its change of intensity, 

 but through its action as a constant force, its effects are 

 apparently of two kinds. One of these is the phenomenon 

 which Loeb (433) calls the tropism, from the Greek word 

 meaning "to turn." In the tropism, the organism takes up 

 a definite position with reference to the action of a force. 

 The phenomenon is therefore connected rather with the 

 spatial aspect of the sense of sight than with its qualitative 

 aspect, and we shall consider it in a later chapter. The 

 other type of effect caused by light as a constant stimulus 

 is that it stimulates or inhibits the general activities of the 

 animal. Thus some animals are restless in strong light, 

 others in darkness. As a result of this influence, the former 

 tend to form collections in the dark, where they remain 

 quietly ; the latter in the light. This influence of a certain 

 intensity of light to stimulate to activity we may call its 

 kinetic effect, or photokinesis. The coelenterate Hydra, 

 for example, has a disposition to come to rest in the more 

 illuminated parts of the vessel containing it (719, 791). 

 Very strong light, however, makes it wander about until 

 it happens to reach a more shaded region. Thus if the 

 animal is subjected to light either above or below a certain 

 "optimum" of intensity, it is restless. A vague uneasiness 

 is the kind of psychic accompaniment to this behavior 

 most naturally suggested. Since repeated strong mechani- 

 cal stimulation also will make the animal wander, nothing 



