890 DISTINCT VISION. [BOOK m. 



brain does not exactly correspond to and is not sharply denned 

 like the retinal area, but gradually fades away into neighbouring 

 sensation-areas. We may imagine two points of the retinal 

 image so far apart that even the extreme margins of their re- 

 spective cerebral sensation-areas do not touch each other in the 

 least ; in such a case there can be no doubt about the two points 

 giving rise to two sensations. We might, however, imagine 

 a second case where two points were just so far apart that their 

 respective sensation-areas should coalesce at their margins, and 

 yet that, in passing from the centre of one sensation-area to the 

 centre of the other, we should find on examination a consider- 

 able fall of sensation at the junction of the two areas ; and in 

 a third case we might imagine the two centres to be so close to 

 each other that in passing from one to the other no appreciable 

 diminution of sensation could be discovered. In the last case 

 there would be but one sensation, in the second there might 

 still be two sensations if the marginal fall were great enough, 

 even though the areas partially coalesced. 



That the ultimate differentiation of the sensations rests with 

 the brain is still more clear in the case of sensations started in 

 the periphery of the retina ; two points of a retinal image might 

 stimulate two cones a considerable distance apart, or several 

 cones, to say nothing of the intervening rods, might be stimu- 

 lated, 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 cere- 

 bral centres. Hence the possibility of increasing by exercise 

 the faculty of distinguishing two sensations, since by use the 

 cerebral sensation-areas become more and more differentiated, 

 though the mosaic of rods and cones fixes for the power of dis- 

 crimination of each individual a limit beyond which exercise 

 cannot carry improvement. This effect of exercise is however 

 shewn in touch even more strikingly than in sight. 



