Before tests, such as have been described, can be presented as conclusive proof of discrimination, it must be shown that the mouse has no preference for the particular brightness which the arrangement of the test requires it to select. That any preference which the mouse to be tested might have for white, rather than black, or for a light gray rather than a dark gray, might be discovered, what may be called preference test series were given before the discrimination tests were begun. These series, two of which were given usually, consisted of ten tests each, with the white alternately on the left and on the right. The mouse was permitted to enter either the white or the black box, as it chose, and to pass through to the nest-box without receiving a shock and without having its way blocked by the glass plate. The conditions of these preference tests may be referred to hereafter briefly as "No shock, open passages." The preference tests, which of course would be valueless as such unless they preceded the training tests, were given as preliminary experiments, in order that the experimenter might know how to plan his discrimination tests, and how to interpret his results. TABLE 7 BRIGHTNESS DISCRIMINATION White-Black, Series II Experimented on No. 5 February 2, 1906 POSITION TEST OF CARDBOARDS RIGHT WRONG 1 White left Right -- 2 White left Right -- 3 White right Right -- 4 White right Right -- 5 White right Right -- 6 White left Right -- 7 White left Right -- 8 White left Right -- 9 White right Right -- 10 White right Right -- Totals 10 0 The results given in the white-black preference tests by ten males and ten females are presented in Table 8. Three facts which bear upon the brightness discrimination tests appear from this table: (1) black is preferred by both males and females, (2) this preference is more marked in the first series of tests than in the second, and (3) it is slightly stronger for the first series in the case of females than in the case of males. TABLE 8 WHITE-BLACK PREFERENCE TESTS MALES FIRST SERIES SECOND SERIES WHITE BLACK WHITE BLACK No. 10 3 7 3 7 18 5 5 5 5 20 2 8 4 6 152 4 6 6 4 210 4 6 4 6 214 6 4 3 7 220 5 5 3 7 230 4 6 2 8 410 4 6 5 5 420 4 6 9 1 Averages 4.1 5.9 4.4 5.6 FEMALES FIRST SERIES SECOND SERIES WHITE BLACK WHITE BLACK