DISCRIMINATION AND COMPARISON. 537 



easy to make in tlie spheres of liglit-, sound-, and pressure- 

 sensation. . . . Beginning with tlie latter case, 



"We find a surprisingly simple result. The barely sensible ad- 

 dition to the original weight must stand exactly in the same proportion 

 to it, be the same fraction of it, no matter what the absolute value 

 may be of the weights on which the experiment is made. ... As the 

 average of a number of experiments, this fraction is found to be about 

 ^ ; that is, no matter what pressure there may already be made upon 

 the skin, an increase or a diminution of the pressure will be felt, as 

 soon as the added or subtracted weight amounts to one third of the 

 weight originally there." 



Wundt then describes how diftereuces may be observed 

 in the muscular feelings, in the feelings of heat, in those of 

 light, and in those of sound ; and he concludes his seventh 

 lecture (from which our extracts have been made) thus : 



" So. we have found that all the senses whose stimuli we are enabled 

 to measure accurately, obey a uniform law. However various may be 

 their several delicacies of discrimination, this holds true of all, that 

 the increase of the stimulus necessary to produce an increase of the sen- 

 sation hears a constant ratio to the total stimulus. The figures w'hich 

 express this ratio in the several senses may be shown thus in tabular 

 form : 



Sensation of light, y^ 



Muscular sensation, . .^ 



Feeling of pressure. \ 



" " warmth, >• \ 



" " sound, ; 



" These figures are far from giving as accurate a measure as might 

 be desired. But at least they are fit to convey a general notion of the 

 relative discriminative susceptibility of the different senses. . . . The 

 important law which gives in so simple a form the relation of the sen- 

 sation to <^he stimulus that calls it forth was first discovered by the 

 physiologist Ernst Heinrich Weber to obtain in special eases. Gustav 

 Theodor Fechner first proved it to be a law for all departments of sen- 

 sation. Psychology owes to him the first comprehensive investigation 

 of sensations from a physical point of view, the first basis of an exact 

 Theory of Sensibility." 



So much for a general account of what Fechner calls 

 Weber's law. The ' exactness ' of the theory of sensibility to 

 which it leads consists in the supposed fact that it gives 

 the means of representing sensations by numbers. The 

 unit of any kind of sensation will be that increment which, 



