582 PHYSIOLOGY 



the periphery of the retina. Thus in an annular zone round the macular 

 region there is red-green blindness but full appreciation of yellow and blue. 

 Outside this area coloured objects are seen in different shades of grey. More 

 careful experiments show firstly, that there is no hard and fast line limiting 

 the zones, but a gradual diminution of colour* perception on passing in any 

 direction from the centre to the periphery; and secondly, that intensity plays 

 a most important part, an increase being sufficient to effect normal colour 

 perception even in quite peripheral vision. The red-green blindness 

 found at one intensity might be due either to an absence or more probably 

 to a weakness or deficiency of either the red or the green sensations, or to 

 defective appreciation on the part of the higher centres in the brain. In 

 the first case there would be an abnormal shape to the luminosity curve, such 

 as is found, in fact, in red or green blindness, whereas in the second case the 

 luminosity curve would be similar to that of normal vision. Experiments 

 are said to show that the curve is normal, and therefore the cones in the peri- 

 phery must be in every way normal, a supposition which is borne out by the 

 correct appreciation of colour at high intensity. Why then it may be asked is 

 colour vision reduced at the periphery if the cones are normal ? The answer 

 is, I think, firstly that the number of cones is greatly diminished, and secondly 

 that the effective area of the pupil is much less at the periphery than it is at 

 the centre of the retina. We have seen in a previous section (1) that the 

 threshold necessary for the appreciation of colour depends on the size of the 

 area of the retina which is receiving stimulation. The larger the area the 

 lower can the intensity be.- Therefore one unit of intensity falling on 

 ICO cones is equivalent to 100 units falling on one cone. Now consider 

 the relative conditions of the fovea and the periphery ; at the fovea let us 

 suppose there to be 100 units of intensity falling on 100 cones, then at the 

 periphery there will be but 50 units (because the effective area of the pupil 

 is less owing to the rays entering obliquely), and these will fall on perhaps 

 two cones only. Whereas in the first case there are 10,000 cone-units, in the 

 second there are 100 cone-units only, and it is therefore to be expected that 

 appreciation of colour would be decreased in the same way as it is at the fovea 

 under reduced illumination. 1 That blue is perceived at a greater angle than 

 yellow is probably due to the greater refraction of blue than yellow rays when 

 they strike the eye surfaces obliquely. For a blue ray and a yellow ray to 

 meet on the retina, the angle subtended by the blue must be 5 per cent, larger 

 than that of the yellow. 



1 The above explanation does not however adequately account for the fact that 

 peripheral vision does not lose its colour appreciation for blue and yellow so readily 

 as it does that for red and green. The phenomena of peripheral vision still require 

 further investigation. 



