THE OUTER AND INNER WORLDS 225 



a complicated mechanism giving rise to 

 different kinds of sensations. In recent years 

 it has been more and more clearly shown 

 that in all the terminal organs of the senses 

 there is differentiation to a degree at one time 

 unsuspected. 



133. It is important also to observe that 

 there seems to be no correspondence between 

 the physical nature of the stimulus and the 

 sensation. They are absolutely unlike. We 

 have acquired knowledge of the various stimuli, 

 say light or sound, by observation, experiment, 

 hypothesis and theory, but there is no simi- 

 larity between certain wave lengths of the 

 ether and a sensation of violet, nor between the 

 varying pressure of a wave in the air and the 

 sensation of a musical tone. The sound of an 

 orchestra may be represented mathematically 

 by a curve, and physically by variations in air 

 pressures, but the psychical effect is quite a 

 different thing. We pass at this point from 

 the region of the physical and physiological 

 (both may ultimately be the same) into the 

 still more occult region of the psychical. 



