FECHNER' S PSYCHOPHYSICAL LAW. 933 



The comparison of one sensation with a second of similar quale, hut of dis- 

 similar quantum, involves more than the mere neural process concerned 

 with a simple sensation. From the very outset it works with ideas based 

 on perceptions, these latter themselves elaborated from sensory impressions. 

 The " law " expresses a striking relation found to hold approximately between 

 the quantity of physical action at one end of the complex chain and the 

 psychical action, as examined by introspective comparison, at the other. 

 Moreover, the process of " attention," variable as it is, has to be used to 

 focus the perceptions, which are therefore apperceptions of variable intensity. 

 On the other hand, the conditions appear simpler in so far that by the "law 

 of relativity " every sensation is ab initio in some sort a judgment and has 

 implicit the terms of a comparison. 



If it be assumed that the just noticeable difference of quantity of stimulus 

 always evokes in a given sense the same quantity of sensation, and if it be 

 further assumed that the " quantity " of sensation evoked by the liminal 

 intensity of the stimulus is the same quantity as that evoked by each liminal 

 difference of intensity, it may be said that the intensity of sensation increases 

 in arithmetic progression when the external stimulus increases in geometrical. 

 The intensity of the sensation varies as the logarithm of the stimulus 

 (Fechner). It has been urged that, as a logical consequence of such 

 assumptions, when 100 grms. is laid on the palm, and to it another 100 

 grms. added, and when, secondly, instead of 100 grms. a weight of 200 grms. is 

 placed there, and to it another of 200 grms. added, the two weights added must 

 in the two cases appear of equal weight ] l but in fact they are felt to be 

 different. 



The so-called " psychophysical law " (Fechner) is an attempt to supply an 

 absolute measure for sensation. It has yet to be shown, however, that it 

 introduces anything more than an arbitrary scale of measurement. Stimulus (x) 

 and sensation (y) are, somewhat remotely, connected as cause and effect, x is 

 a quantity capable of measurement, and of numerical expression. Of y no 

 measure in the ordinary sense can be found, and the statement that one 

 value of y was double of another, would have, apart from special conventions, 

 no definite meaning. Sensation like temperature is no doubt variable in 

 degree, but changes of sensation, unlike changes of temperature, do not, so far 

 as known, produce measurable changes in any object. Anything comparable 

 with a thermometric scale cannot, therefore, be constructed for sensations. So 

 long as there is no independent measure of sensation, the "psycho-physical 

 law " cannot be spoken of as either a right or wrong interpretation of fact, 

 although it may offer a convenient mode of representing, numerically, intensity 

 of sensation. 



The strength of an adequate stimulus may — its duration remaining the 

 same — alter in three ways. It can act with more or less intensity on a certain 

 definite number of end-organs ; it can act with an unvarying intensity upon a 

 more or a less extensive number of end-organs ; or, by combination of both the 

 preceding, its intensiveness and extensiveness may simultaneously and independ- 

 ently vary. It is noteworthy that the physiological results are in the three cases 

 less dissimilar than might have been thought. Paths of conduction in the 

 nervous system communicate in virtue of the branching of nerve cells, and across 

 nerve cell junctions. The endings of nerve channels frequently embouch into 

 common mechanisms, — that is, their areas of end-distribution overlap. Further, 

 the impulses excited in nervous reactions tend to " irradiate " in nerve centres. 

 By " irradiation " or " overflow," variation in pure intensity of stimulus pro- 

 duces some of the same effects as variation of the stimulus in extensiveness. 

 In virtue of " overlap," variation in the mere spatial extension of the stimulus 



1 Hering, Sitzungsb. d. k. Akad. d. Wissensch., Wien, 1875, Bd. lxxii. Abth. 3, S. 310. 

 This communication contains also the experiments testing Weber's law, carried out under 

 Hering's direction by Lowit and Biedermann, 



