H4 The Dancing Mouse 



the Weber's law test, transmitted light was used, in an ap- 

 paratus which will be described later. Either of these meth- 

 ods might have been used for the solution of both problems. 

 Which of them is the more satisfactory is definitely decided 

 by the results which make up the material of this chapter. 

 Under natural conditions the dancer probably sees objects 

 which reflect light more frequently than it does those which 

 transmit it; it would seem fairer, therefore, to require it to 

 discriminate surfaces which differ in brightness. This the 

 use of gray papers does. But, on the other hand, gray papers 

 are open to the objections that they nay not be entirely 

 colorless (neutral), and that their brightness values cannot, 

 be changed readily by the experimenter. As will be made 

 clear in the subsequent description of the experiments with 

 transmitted light, neither of these objections can be raised 

 in connection with the second method of experimentation. 



To determine the delicacy of discrimination with reflected 

 light it is necessary to have a series of neutral grays (colorless) 

 whose adjacent members differ from one another in bright- 

 ness by less than the threshold of discrimination of the animal 

 to be tested. A series which promised to fulfill these con- 

 ditions was that of Richard Nendel of Berlin. It consists 

 of fifty papers, beginning with pure white, numbered i, and 

 passing by almost imperceptible steps of decrease in bright- 

 ness through the grays to black, which is numbered 50. For 

 the present we may assume that these papers are so nearly 

 neutral that whatever discrimination occurs is due to bright- 

 ness. The differences between successive papers of the series 

 are perceptible to man. The question is, can they, under 

 favorable conditions of illumination, be perceived by the 

 dancer ? 



On the basis of the fact that the dancer can discriminate 

 between white and black, two grays which differed from one 



