CHAPTER VII 



Sensory Discrimination: Vision 

 § 38. Change of Light Intensity as a Stimulus 



In this chapter we shall omit all references to the func- 

 tion of vision as a spatial sense, that is, as giving rise to 

 perceptions of form, size, distances, and direction. 



It appears that light may act upon living beings either 

 as a continuous or as an interrupted stimulus. That is, 

 light maintained steadily at a constant intensity produces 

 responses in organisms, and there are also reactions when 

 the intensity of the light is suddenly altered, in the direc- 

 tion either of brightening or of darkening. Most physical 

 forces act as stimuli only when they change in some way : 

 an unchanging environment fails to call forth response. 

 We may briefly survey the facts which point to the exist- 

 ence in animals of reactions to changes in light intensity. 



Among the Protozoa, Amoeba, when subjected to sud- 

 den changes in light intensity, checks its movement at the 

 point where the light falls. This is just what happens when 

 a mechanical stimulus is applied, and offers no evidence 

 of a specific light sensation (378). Similarly, various 

 ciliate and flagellate Protozoa give their ordinary negative 

 or avoiding reaction to changes in light intensity ; some of 

 them make it on passing from a region of less to one of 

 greater illumination, and thus "seek" the darker regions, 

 while others give it when undergoing a change in the re- 

 verse direction, and thus tend to remain in lighter regions. 



^35 



