220 PRINCIPLES OF PHYSIOLOGY 



but not the retina. As we pass upwards, 

 either by increasing the intensity or, as in 

 viewing a spectrum, by increasing the number 

 of vibrations, sensation continues, and it may 

 vary in intensity, or in quality, or in both. 



129. The relation between the strength of 

 the stimulus applied to an organ of sense and 

 the intensity of the sensation has been investi- 

 gated. It is found that the intensity of the 

 sensation increases with an increase in the 

 strength of the stimulus, but in a peculiar way. 

 It is not in direct proportion. For example, 

 doubling or trebling the strength of the stimu- 

 lus does not double or treble the intensity of 

 the sensation, but the latter increases by 

 smaller and smaller increments until no 

 difference in the intensity of the sensation 

 can be observed. Thus, in a very intense 

 light the additional light of a candle may not 

 be perceived Refinements of this law have 

 been studied, but the general principle is as 

 above stated. It is evident that there is 

 thus a protective action against injury from 

 excessive stimulation. 



130. With regard to vibrations, a sensation 



