As is shown by the results in Table 28, no combination of brightnesses rendered correct choice impossible in the case of the blue-red tests which are now to be described. Choice was extremely difficult at times, even more so perhaps than the table would lead one to suppose, and it is quite possible that color played no part in the discrimination. But that brightness difference in the colors was not responsible for whatever success these mice attained in selecting the right box is proved by the brightness-without-color series which follows series II 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 0 in one box (darkness) and in the other red of a brightness of 3 candle meters. Despite the fact that they had been perfectly trained in _blue-red tests_ to avoid the red, neither of the mice seemed able to discriminate the red from the darkness and to avoid it. This was followed by a series in which the brightness of both the blue and the red was varied between 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 distinguish 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 learned to avoid red. Therefore we are still confronted with the question, can they see colors? TABLE 29 VISUAL CHECK TESTS With the Electric-boxes Precisely Alike Visually No. 151 No. 152 SERIES DATE RIGHT WRONG RIGHT WRONG