1 1 6 The Dancing Mouse 



choose the brighter of the two if they were able to detect 

 the difference. As a matter of fact this did not always occur ; 

 some individuals had to be trained to discriminate gray No. 10 

 from gray No. 20. As soon as an individual had been so 

 trained that the ability to choose the lighter of these grays was 

 perfect, it was tested with No. 10 in combination with No. 15. 

 If these in turn proved to be discriminate, No. 10 could be 

 used with No. 14, with No. 13, and so on until either the limit 

 of discrimination or that of the series had been reached. 



That it was not necessary to use other combinations than 

 10 with 20, and 10 with 15 is demonstrated by the results 

 of Table 13. Mouse No. 420, whose behavior was not essen- 

 tially different from that of three other individuals which 

 were tested for gray discrimination, learned with difficulty 

 to choose gray No. 10 even when it was used with No. 20. 

 Two series of ten tests each were given to this mouse daily, 

 and not until the twentieth of these series (200 tests) did he 

 succeed in making ten correct choices in succession. Imme- 

 diately after this series of correct choices, tests with grays 

 No. 10 and No. 15 were begun. In the case of this amount 

 of brightness difference twenty series failed to reveal discrimi- 

 nation. The average number of right choices in the series 

 is slightly in excess of the mistakes, 5.8 as compared with 4.2. 



From the experiments with gray papers we may conclude 

 that under the conditions of the tests the amount by which 

 Nendel's gray No. 10 differs in brightness from No. 20 is near 

 the threshold of discrimination, or, in other words, that the 

 difference in the brightness of the adjacent grays of Figure 16 

 is scarcely sufficient to enable the dancer to distinguish them. 



This result of the tests with gray papers surprised me very 

 much at the time of the experiments, for all my previous ob- 

 servation of the dancer had led me to believe that it is very 

 sensitive to light. It was only after a long series of tests with 



