10 PHYSIOLOGY CHAP. 



necessary to add the same fraction of the weight (average T V, 

 according to Weber), whatever its absolute value whether in 

 ounces, pounds, grammes, or kilogrammes. 



Later observations by a number of investigators have proved 

 that, within certain limits, Weber's law is approximately valid 

 for all the different modalities of sensation, provided stimuli of 

 medium strength are employed. On the other hand, there are 

 more or less marked exceptions to the law when the stimulus is 

 too strong or too weak. Generally speaking, Weber's law expresses 

 a fact of great empirical importance, but has no claim to be a 

 method of absolute measurement of sensation, or of exact deter- 

 mination of the ratio between sensation and stimulus. 



The same cannot be said for the so-called " psycho-physical 

 law" which Fechner (1860) formulated as a larger generalisa- 

 tion from Weber's law. According to Fechner, if the increase 

 of the sensation is proportional to the increase of the stimulus 

 divided by the absolute intensity of the excitation, the 

 sensations will stand in the same ratio to the stimuli as do 

 logarithms to their numbers. Let S be the sensation, R the 

 stimulus, C the constant represented by the liminal difference, 

 and Fechner's "formula of psycho -physical measurement" is 

 obtained : S = C log R, i.e. sensation is proportional to the 

 logarithm of the stimulus. 



Fechner's theoretical interpretation of Weber's law is open to 

 serious objections. Fechner assumes that the value of the 

 liminal difference remains the same at all points of the scale 

 ($ A = constant), while experiment shows that Weber's law only 

 holds good within certain limits, and that the value of $ A alters 

 at the extremes of the strength of stimulus. Fechner further 

 assumes that the smallest appreciable increase of a sensation 

 represents its unit of magnitude, and that all sensations result 

 from different sums of such units, which is a purely arbitrary 

 interpretation of Weber's law, supported neither from introspective 

 investigation nor from physiological observation. It is one thing 

 to state with Weber that the relation between the appreciable 

 increase of a stimulus and its absolute magnitude is constant 

 within certain limits and quite another to say with Fechner that 

 every appreciable increment of stimulus invariably excites a 

 sensation of the same value, and that these sensations together 

 summate into a complex whole. The idea of giving a numerical 

 measure of sensations is, according to William James, purely and 

 simply a mathematical speculation upon eventual possibilities, 

 which has never found any practical application. The psycho- 

 physical law will always remain a fossil in the history of 

 psychology. 



III. Up to this point we have discussed sensations, and their 

 different modalities, qualities, and intensities. But psychologists 



