The Sense of Sight: Brightness Vision 125 



The results which were obtained in the case of this second 

 standard, the value of which was 80 hefners, are strikingly 

 different from those for the 20 hefner standard. Naturally 

 I began the tests with this new standard by making the dif- 

 ferences the same as those for which determinations had been 

 made in the case of the 20 standard. Much to my surprise 

 only 6 per cent of errors resulted when the difference in illumi- 

 nation was .53. I finally discovered that about .19 difference 

 (about one fifth) could be discriminated with that degree of 

 accuracy which is indicated by 25 per cent of mistakes. 



So far as I could judge from the results of determinations 

 for the 20 and the 80 hefner standards, Weber's law does 

 not hold for the dancer. With the former a difference of 

 almost one half was necessary for discrimination; with the 

 latter a difference of about one fifth could be perceived. But 

 before presenting additional results I should explain the con- 

 struction of Table 14, and comment upon the number of 

 experiments which constitutes a set. 



The table contains the condensed results of several weeks 

 of difficult experimentation. From left to right the columns 

 give the date of the initial series of a given set of experiments,- 

 the number of experiments in the set, the value of the standard 

 light in hefners, the value of the variable light, the difference 

 between the lights in terms of the standard (the variable was 

 always less than the standard), and the percentage of errors 

 or wrong choices. Very early in the investigation I discovered 

 that one hundred tests with any given values of the lights 

 sufficed to reveal whatever discriminating ability the mouse 

 possessed at the time. In some instances either the presence 

 or the lack of discrimination was so clear, as the result of 50 

 tests (first series), that the second series of 50 was not given. 

 Consequently in the table the number of tests for the vari- 

 ous values of the lights is sometimes 100, sometimes 50. 



