With any form of the method of just noticeable differences in which the subject himself alters the stimulus of comparison, there is liability to serious error from the subject’s inclination to judge in terms of movement. When he has found that a certain hand-movement has made the stimulus of comparison just noticeably greater or less than the standard, he will expect the same movement to make it just noticeably greater or less again. He will be all the more tempted to judge in terms of hand-movement from the fact that he has been all his life forming estimates of space in terms of the sensations produced by movement, and has probably never thought of taking pains to compare the intensity of two odors. This tendency varies much in different subjects. Its presence may be suspected when the mean variation of a series is very small. Fortunately, it acts in such a way as rather to conceal the operation of Weber's law, if applicable, than to make it appear applicable if it were not. If, for example, one finds Δr to be 5 mm., for a standard of 20 mm., and by repeating the series of movements, obtains the same value of Δr for a standard of 40 mm., Δr⁄r will be 1⁄4 in the one case, and 1⁄8 in the other.
As a matter of fact our results offer evidence for the law which is strong to an almost suspicious degree. Yet it is not probable that a trained subject would, or that an untrained subject could deliberately alter his movements, when the stand- was varied, so as to keep the value of Δr⁄r approximately the same, and it is absolutely impossible that twelve subjects out of thirteen should all do so. Such a procedure would argue a miraculous combination of psychophysical knowledge, accurate memory, industry and malice.
We also made some attempt to test the applicability of the method of right and wrong cases. At the time we tried it, which was early in the course of our experiments, we found it utterly impracticable, The fact that more than half the mistakes were made in thinking the second stimulus weaker than the first or equal to it, would indicate that exhaustion was the disturbing factor. Since, however, the subject seems genuinely to recognize the stimulus of comparison in the gradation-methods as greater or less than the standard, it is probable that the difficulty with the method of right and wrong cases is largely the utter confusion it produces in his mind. Most persons are not used to smelling attentively and have to “learn” a given smell-intensity.
