In that the nomenclature of intensity of stimulation is based upon the intensity of response, the question arises as to the _relation between the intensity of stimulus and the amount of response_. It is well known that this question has met in one special field of physiology with a very detailed and comprehensive treatment. I allude to the teaching concerning sensation. _Ernst Heinrich Weber_[21] first called attention to the relation between increase in sensation and that of the stimulus in the case of the sense of touch. His observations, which have been formulated into “_Weber’s law_,” have been the object of animated discussion. A presentation of this law is the following: “The amount of pressure necessary to produce a perceptible increase of sensation always bears the same ratio to the amount of the stimulus already applied.” [21] _Weber_: “Annotationes anatomicæ et physiologicæ.” Lips. 1851. The same: “Der Tastsinn und das Gemeingefühl,” in Wagner’s Handwörterbuch d. Physiologie Bd. III. 2. Braunschweig 1846. If in accordance with _Ziehen_[22] we designate the relative increase in pressure to that already applied, which is necessary to produce a perceptible increase in sensation, as the _threshold of relative differentiation_, we can formulate the law in the simplest way thus: The _relative threshold of differentiation is constant_. _Fechner_,[23] who indeed attempted to apply this law, applicable to the sense of pressure, to all the other special senses, has given us a mathematical formula, based on the assumption that the just perceptible increase of sensation has the same value at all levels. By this assumption he was able to establish for the first time a relation between the intensity of sensation and that of stimulus, for it follows that “_the sensation increases in intensity in arithmetical progression, whereas the intensity of the stimulus increases in geometrical progression_.” From this _Fechner has_ worked out a psychophysical formula, which today is generally termed the _Fechner law_. This is the law: _The intensity of sensation varies with the logarithm of the intensity of the stimulus._ [22] _Ziehen_: “Leitfaden der physiologischen Psychologie in 15 Vorlesungen.” VI Auflage. Jena 1902. [23] _Fechner_: “Elemente der Psychophysik.” Leipzig 1860. 2 Auflage 1889.