122 The Dancing Mouse 



For reasons which will soon appear, Weber's law tests 

 were made with only one dancer. This individual, No. 51, 

 had been thoroughly trained in white- black discrimination 

 previous to the experiments in the apparatus which is repre- 

 sented in Figure 17. Having given No. 51 more than two 

 hundred preliminary tests in the Weber's law apparatus with 

 the electric -boxes sufficiently different in brightness to enable 

 her to discriminate readily, I began my experiments by trying 

 to ascertain how much less the value of the illumination ot 

 one electric-box must be in order that it should be discrim- 

 inable from a value of 20 hefners in the other electric-box. 

 In recording the several series of tests and their results here- 

 after, I shall state in Hefner units the value of the fixed or 

 standard light and the value of the variable light, the difference 

 between the two in terms of the former, and the average 

 number of wrong choices in per cent. 



With the lamps so placed that the difference in the illumi- 

 nation of the two electric-boxes was .53 of the value of the 

 standard, that is about one half, No. 51 made twenty wrong 

 choices in one hundred, or 20 per cent. When the difference 

 was reduced to .36 (one third) the number of errors increased 

 to 36 per cent, and with an intermediate difference of .48 

 there were 26 per cent of errors (see Table 14). 



Are these results indicative of discrimination, or are the 

 errors in choice too numerous to justify the statement that 

 the dancer was able to distinguish the boxes by their differ- 

 ence in brightness? Evidently this question cannot be 

 answered satisfactorily until we have decided what the per- 

 centage of correct choices should be in order that it be ac- 

 cepted as evidence of ability to discriminate, or, to put it in 

 terms of errors, what percentage of wrong choices is indica- 

 tive of the point of just perceivable difference in brightness. 

 Theoretically, there should be as many mistakes as right 



