The Sense of Sight: Color Vision 175 



whatever success these mice attained in selecting the right 

 box is proved by the brightness-without-color series which 

 follows series n of the table. Neither No. 2 nor No. 205 

 showed preference for the lighter or the darker box. At 

 the end of the sixteenth blue-red series, I was convinced that 

 one of two conclusions must be drawn from the experiment : 

 either the dancers possess a kind of blue-red vision, or red 

 is of such a value for them that no brightness of visible green 

 or blue precisely matches it. 



The latter possibility was further tested by an experiment 

 whose results appear in series 17 to 22 inclusive, of Table 28. 

 The conditions of series 17 were a brightness value of o in 

 one box (darkness) and in the other red of a brightness of 

 3 candle meters. Despite the fact that they had been per- 

 fectly trained in blue-red tests to avoid the red, neither of 

 the mice seemed able to discriminate the red from the dark- 

 ness and to avoid it. This was followed by a series in which 

 the brightness of both the blue and the red was varied be- 

 tween 3 and 3650 candle meters, with the striking result that 

 neither mouse made any mistakes. In series 19 red was 

 used with darkness as in series 17, and again there was a 

 total lack of discrimination. Series 20 was a repetition of 

 series 18, with practically the same result. I then attempted 

 to find out, by increasing the brightness of the red, how great 

 must be its value in order that the dancers should distin- 

 guish it readily from darkness. For the tests of series 21 it 

 was made 72 candle meters, but discrimination did not 

 clearly appear. At 1800 candle meters, as is shown in series 

 22, the red was sufficiently different in appearance from 

 total darkness to enable No. 205 to discriminate perfectly 

 between the two electric-boxes. For No. 2 discrimination 

 was more difficult, but there was no doubt about his ability. 

 It would appear from these tests that the dancers had not 



