§ 4. Limitations of Weber's Law.—We have spoken of Weber's law as if it held good exactly and uniformly for all sensations; but as a matter of fact this is far from being the case. Many deviations and limitations have been discovered by experiment. Verification commonly fails for very high or very low intensities of sensation. In view of the complexity of the operative conditions this is not in the least surprising. Our power of discriminating may be influenced by many factors besides the actual nature of the sensations which we have to compare. The relation between intensity of stimulus and absolute intensity of sensation may, and probably does, depend upon many other conditions than the mere intensity of the stimulus itself. We may suppose the law to be perfectly exact, inas much as it states that unlikeliness between sensations depends upon their relative difference, without supposing that this relative difference is determined only by difference of external stimulation. The special structure of the different senseorgans is probably an important factor. To speak of nothing else, the eye and the ear have sensations of their own due to internal stimulation, which it is difficult to allow for.*

* The treatment of Weber's law in this chapter follows Meinong, Ueber die Bedeutung des Weberschen Gesetzes, etc.