CHAP, in.] , SIGHT. 1221 



to say nothing of the intervening rods, might be stimulated, and 

 yet one sensation only result. 



Thus, the distinction or fusion of visual sensations is ulti- 

 mately determined by the disposition and condition of the cerebral 

 centres. Hence the possibility of increasing by exercise the faculty 

 of distinguishing two sensations, since by use the cerebral sensa- 

 tion-areas become more and more differentiated, though the mosaic 

 of rods and cones fixes for the power of discrimination of each 

 individual a limit beyond which exercise cannot carry improve- 

 ment. This effect of exercise is however shewn in touch even 

 more strikingly than in sight. 



782 



