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 essentially 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. Immediately 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 discrimination. 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. TABLE 13 GRAY DISCRIMINATION The Delicacy of Brightness Discrimination No. 420 GRAYS NOS. 10 GRAYS NOS. 20 AND 20 AND 15 SERIES DATE DATE NO. 10 NO. 2 NO. 10 NO. 15 (RIGHT) (WRONG) (RIGHT) (WRONG) 1 Jan. 26 5 5 Feb. 6 8 2 2 27 8 2 6 5 5 3 28 6 4 7 9 1 4 28 2 8 7 7 3 5 29 1 9 8 5 5 6 29 6 4 8 6 4 7 30 9 1 9 5 5 8 30 7 3 9 6 4 9 31 6 4 10 8 2 10 31 4 6 10 3 7 11 Feb. 1 7 3 11 4 6 12 1 8 2 11 4 6 13 2 7 3 12 7 3 14 2 8 2 12 7 3 15 3 9 1 13 6 4 16 3 9 1 13 4 6 17 4 6 4 14 4 6 18 4 9 1 14 5 5 19 5 6 4 15 5 5 20 5 10 0 15 8 2 Averages 6.6 3.4 5.8 4.2