May 13 100 20 9.4 .53 20 15 100 20 12.8 .36 36 16 100 20 10.8 .46 26 20 50 80 37.6 .53 6 21 50 80 51.3 .36 10 22 100 80 71.1 .11 35 24 100 80 60.0 .25 21 25 100 80 65.0 .19 25 27 100 80 80 0 41 28 50 5 2.5 .50 18 29 50 5 4.0 .20 14 29 100 5 4.5 .10 25 31 50 5 4.25 .15 20 June 1 50 5 4.85 .03 48 2 50 20 15.0 .25 16 3 50 20 17.4 .13 22 3 100 20 18.0 .10 22 4 100 80 72.0 .10 18 5 100 5 4.5 .10 12 7 100 5 4.67 .067 46 8 50 80 74.67 .067 56 9 50 20 18.67 .067 44 If we apply this rule to the results of the first tests, reported above, it appears that a standard of 20 hefners was distinguished from a variable of 9.4 hefners (.53 difference), for the percentage of errors was only 20. But in the case of a difference of .36 in the illuminations lack of discrimination is indicated by 36 per cent of errors. A difference of .46 gave a frequency of error so close to the required 25 (26 per cent) that I accepted the result as a satisfactory determination of the just perceivable difference for the 20 hefner standard and proceeded to experiment with another standard value. The results which were obtained in the case of this second standard, the value of which was 80 hefners, are strikingly different from those for the 20 hefner standard. Naturally I began the tests with this new standard by making the differences the same as those for which determinations had been made in the case of the 20 standard. Much to my surprise only 6 per cent of errors resulted when the difference in illumination was .53. I finally discovered that about .19 difference (about one fifth) could be discriminated with that degree of accuracy which is indicated by 25 per cent of mistakes. So far as I could judge from the results of determinations for the 20 and the 80 hefner standards, Weber's law does not hold for the dancer. With the former a difference of almost one half was necessary for discrimination; with the latter a difference of about one fifth could be perceived. But before presenting additional results I should explain the construction of Table 14, and comment upon the number of experiments which constitutes a set.