CHAPTER VII 

 SENSORY DISCRIMINATION: VISION 



40. Some Problems connected with Vision 



IN this chapter we shall consider one aspect only of the 

 reactions of animals to light stimulation ; namely, the question 

 whether such stimulation produces in the possible conscious- 

 ness of a given animal any sensations qualitatively unlike those 

 accompanying other forms of stimulation, and if so, how 

 many such specifically visual sensations, qualitatively differ- 

 ent from each other, the animal may be supposed to be capable 

 of receiving. The spatial aspect of vision will for the present 

 be neglected. 



Even with this restriction, the photic reactions of animals 

 present a series of problems of enormous complexity. One 

 especially difficult question is, it is true, postponed : the ques- 

 tion as to just what happens when an animal seeks or avoids 

 light. The so-called orientation of animals, that is, their 

 assumption of a definite position with reference to a force 

 acting upon them at a certain point, is a subject more closely 

 connected with spatial than with qualitative discrimination; 

 and though, as we shall see, the seeking or avoiding of 

 light by an animal by no means always involves orientation 

 of the body, yet the complex distinctions that have to be drawn 

 in connection with this subject will be more fully discussed 

 under the head of orienting reactions. But puzzles enough are 

 left for the present chapter. What, for instance, is the mean- 

 ing of the fact that the rays beyond the violet end of the spec- 

 trum, invisible to us, produce effects upon certain animals? 



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