CHAPTER VIII 



THE SENSE OF SIGHT : BRIGHTNESS VISION (Continued) 



SINCE the ability of the dancer to perceive brightness has 

 been demonstrated by the experiments of the previous chap- 

 ter, the next step in this investigation of the nature of vision 

 is a study of the delicacy of brightness discrimination, and 

 of the relation of the just perceivable difference to brightness 

 value. Expressed in another way, the problems of this por- 

 tion of the investigation are to determine how slight a dif- 

 ference in brightness enables the dancer to discriminate one 

 light from another, and what is the relation between the 

 absolute brightnesses of two lights and that amount of dif- 

 ference which is just sufficient to render the lights distin- 

 guishable. It has been discovered in the case of the human 

 being that a stimulus must be increased by a certain definite 

 fraction of its own value if it is to seem different. For 

 brightness, within certain intensity limits, this increase must 

 be about one one- hundredth ; a brightness of 100 units, for 

 example, is just perceivably different from one of 101 units. 

 The formulation of this relation between the amount of a 

 stimulus and the amount of change which is necessary that 

 a difference be noted is known as Weber's law. Does this 

 law, in any form, hold for the brightness vision of the danc- 

 ing mouse? 



Two methods were used in the study of these problems. For 

 the first problem, that of the delicacy of brightness discrimina- 

 tion, I first used light which was reflected from gray papers, 

 according to the method of Chapter VII. For the second, 

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