CHAP. IL] SIGHT. 523 



length of 5'36^ in the retinal image 1 , and one of 50 seconds to 

 3-65 p. 



In the human eye 50 cones may be counted along a line of 

 200 fj, in length drawn through the centre of the yellow spot ; this 

 would give 4 //. for the distance between the centres of two ad- 

 joining cones in the yellow spot, the average diameter of a cone at 

 its widest part being 3 //, and there being slight intervals between 

 neighbouring cones. Hence if we take the centre of a cone as the 

 centre of an anatomical retinal area, these anatomical areas cor- 

 respond very fairly to the physiological visual areas as determined 

 above. That is to say, 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 /j, apart that they 

 cease to give rise to two distinct sensations. It must be remem- 

 bered, however, that the fusion or distinction of the sensations is 

 ultimately determined by the brain and not by the retina. Two 

 points of the retinal image less than 4 //, apart might lie both 

 within the area of a single cone; but the reason why, under such 

 circumstances, they give rise to one sensation only is not because 

 one cone-fibre only is stimulated. 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 that 4 fi apart ; in such a case 

 two cone-fibres would be stimulated, and yet only one sensation 

 would be produced. So also in the less sensitive peripheral parts 

 of the retina two points of the retinal image might stimulate two 

 cones a considerable distance apart, and yet give rise to one sensa- 

 tion only. 



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 



1 By fj. is meant one-thousandth of a millimetre. 



