The Sense of Sight : Brightness Vision 123 



choices, 50 per cent of each, when the two electric-boxes are 

 equally illuminated (indiscriminable), but in practice this 

 does not prove to be the case because the dancer tends to 

 return to that electric-box through which in the previous 

 test it passed safely, whereas it does not tend in similar fash- 

 ion to reenter the box in which it has just received an elec- 

 tric shock. The result is that the percentage of right choices, 

 especially in the case of series which have the right box in 

 the same position two, three, or four times in succession, 

 rises as high as 60 or 70, even when the visual conditions 

 are indiscriminable. Abundant evidence in support of this 

 statement is presented in Chapters VII and IX, but at 

 this point I may further call attention to the results of an 

 experiment in the Weber's law apparatus which was made 

 especially to test the matter. The results appear under the 

 date of May 27 in Table 14. In this experiment, despite 

 the fact that both boxes were illuminated by 80 hefners, the 

 mouse chose the standard (the illumination in which it was 

 not shocked) 59 times in 100. In other words the percentage 

 of error was 41 instead of 50. It is evident, therefore, that 

 as low a percentage of errors as 40 is not necessarily indicative 

 of discrimination. Anything below 40 per cent is likely, 

 however, to be the result of ability to distinguish the brighter 

 from the darker box. To be on the safe side we may agree 

 to consider 25 wrong choices per 100 as indicative of a just 

 perceivable difference in illumination. Fewer mistakes we 

 shall consider indicative of a difference in illumination which 

 is readily perceivable, and more as indicative of a difference 

 which the mouse cannot detect. The reader will bear in mind 

 as he examines Table 14 that 25 per cent of wrong choices 

 indicates the point of just perceivable difference in brightness. 

 If we apply this rule to the results of the first tests, reported 

 above, it appears that a standard of 20 hefners was distin- 



