The table contains the condensed results of several weeks of difficult experimentation. From left to right the columns give the date of the initial series of a given set of experiments, the number of experiments in the set, the value of the standard light in hefners, the value of the variable light, the difference between the lights in terms of the standard (the variable was always less than the standard), and the percentage of errors or wrong choices. Very early in the investigation I discovered that one hundred tests with any given values of the lights sufficed to reveal whatever discriminating ability the mouse possessed at the time. In some instances either the presence or the lack of discrimination was so clear, as the result of 50 tests (first series), that the second series of 50 was not given. Consequently in the table the number of tests for the various values of the lights is sometimes 100, sometimes 50. After finishing the experiments with the 80 standard on May 27 (see table) I decided, in spite of the evidence against Weber's law, to make tests with 5 as the standard, for it seemed not impossible that the lights were too bright for the dancer to discriminate readily. I even suspected that I might have been working outside of the brightness limits in which Weber's law holds, if it holds at all. The tests soon showed that a difference of one tenth made discrimination possible in the case of this standard. If the reader will examine the data of the table, he will note that a difference of .20 gave 14 per cent of mistakes; a difference of .03, 48 per cent. Evidently the former difference is above the threshold, the latter below it. But what of the interpretation of the results in terms of Weber's law? The difference instead of being one half or one fifth, as it was in the cases of the 20 and 80 standards respectively, has now become one tenth. Another surprise and another contradiction! Had these three differences either increased or decreased regularly with the value of the standard I should have suspected that they indicated a principle or relationship which is different from but no less interesting than that which Weber's law expresses. But instead of reading 5 standard, difference one tenth; 20 standard, difference one fifth; 80 standard, difference one half: or 5 standard, difference one half; 20 standard, difference one fifth; 80 standard, difference one tenth: they read 5 standard, difference one tenth; 20 standard, difference one half; 80 standard, difference one fifth. What does this mean? I could think of no other explanation than that of the influence of training. It seemed not impossible, although not probable, that the mouse had been improving in ability to discriminate day by day. It is true that this in itself would be quite as interesting a fact as any which the experiment might reveal.