258 THE SPECIAL SENSES. 



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. 111). 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. From the physiological standpoint it is 

 important to bear in mind, as has been emphasized by Waller,* that 

 several steps intervene between the action of the external stimulus 

 and the production of the conscious sensation. The external stim- 

 ulus acts first on the end-organ, this in turn upon the sensory nerve 

 fiber, producing a nerve impulse which finally in the brain gives the 

 conscious reaction. It is a question, therefore, whether the logarith- 

 mical relation of the stimulus holds between it and the reaction of the 

 end-organ or between the internal stimulus that is, the sensory 

 nerve impulse and the psychical reaction. This author has given 

 some facts obtained by recording the action current in the optic 

 nerve, the retina being stimulated by known intensities of light, 

 which indicate that the relation observed is between the external 

 stimulus and the internal stimulus, that is, the sensory nerve 

 impulse. 



* Waller, "Brain," 201, 1895. 



