CLASSIFICATION OF THE SENSES. 273 



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 ^, 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. 



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