S-2 DISTINCT VISION. [BOOK in. 



anatomical areas correspond very fairly in the region of distinct 

 vision to the physiological visual areas just spoken of. If two 

 points of the retinal image are less than 4 //, apart, they may both 

 lie within the area of a single cone ; and it is just when they are 

 less than about 4 JJL apart that they cease to give rise to two 

 distinct sensations. It must be remembered, however, that the 

 fusion or distinction of the sensations is ultimately determined 

 by the brain. The retinal area must be carefully distinguished 

 from the sensational unit, for the sensation is a process whose 

 arena stretches from the retina to certain parts of the brain, and 

 the circumscription of the sensational unit, though it must begin 

 as a retinal area, must also be continued as a cerebral area, the 

 latter corresponding to, and being as it were the projection of, the 

 former. Two points of the retinal image less than 4 ^ apart 

 might lie both within the area of a single cone ; but the reason 

 w T hy, under such circumstances, they give rise to one sensation 

 only is not because one cone-fibre only is stimulated. For, two 

 points of a retinal image might lie, one on the area of one cone 

 and another on the area of an adjoining cone, and still be less than 

 4 /A apart ; in such a case two cone-fibres would be stimulated ; 

 and yet only one sensation would be produced. 



In the case where the two points lie entirely within the area of 

 a single cone, it is exceedingly probable that, even if the adjacent 

 cones or cone-fibres in the retina are not at the same time stimu- 

 lated, impulses radiate from the cerebral ending of the excited 

 cone into the neighbouring cerebral endings of the neighbouring 

 cones ; in other words, the sensation-area in the brain does not 

 exactly correspond to and is not sharply defined 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 respective 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 considerable 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. 



