CLASSIFICATION OF THE SENSES. 277 



in sensation represent actually equal amounts of sensation. Ac- 

 cepting this assumption, we can express the relationship between 

 stimulus and sensation as determined by Weber's experiments by 

 saying that for the sensation to increase by equal amounts, that is, 

 by arithmetical progression, the stimulus must vary according 

 to a certain factor, that is, by geometrical progression. The 

 sensation may be regarded as a geometrical function of the 

 stimulus. If the relation between stimulus and sensation is repre- 

 sented as a curve in which the ordinates express the sensation in- 

 creasing by equal amounts, and the abscissas the corresponding 

 stimuli increasing at each interval by J, a result is obtained such as 

 is represented in the accompanying figure (Fig. 118). A curve of 

 this kind is a logarithmical curve, and Fechner expressed the rela- 

 tionship between stimulus and sensation in what has been called the 

 psychophysical law, namely, that the sensation varies as the 

 logarithm of the stimulus. 



