VISION OF DIFFERENT REGIONS OF THE RETINA. 1085 



There is considerable difference of opinion on the question of the exact 

 limits of the fields for different colours, but there is now fairly general 

 agreement in placing the limits for blue and yellow distinctly outside 

 those for red and green. Nearly all agree that the limits are not fixed, 

 and are greatly influenced by intensity and the other factors mentioned. 



Another important feature of peripheral colour vision is the change 

 in tone which a coloured light appears to undergo as it passes over the 

 retina from centre to periphery. Here again there is fairly general 

 agreement that blue and yellow become colourless without undergoing 

 any change in tone, while reds and greens pass through a stage in which 

 they are seen as yellow or blue before becoming colourless. Violet is 

 seen as blue, orange as yellow. Some observers, however, have seen 

 yellow change to green peripherally. 



On passing a large number of pigment colours over the retina from 

 periphery to centre, Hess 1 found that nearly all showed change of colour- 

 tone. The colours fell into two groups : one group appeared yellow and 

 the other blue in the periphery. Each of these groups could be again 

 divided into those which appeared red and those which appeared green as 

 they approached the centre. In addition to these four groups, Hess found 

 four colours which showed no change in colour-tone : these four colours 

 formed two complementary pairs, yellow and blue, red and green. The 

 red and green which did not change in tone were found with more 

 difficulty than the yellow and blue, nearly all the pigments used 

 showing a yellow or blue tone at some time in their course over the 

 retina. The red which showed no change was bluer than the red of the 

 spectrum. With spectral colours the same general changes were 

 observed, but only three kinds of light could be found which showed no 

 change in tone, in the yellow at 574 - 5 A, in the green at 495 A, and in the 

 blue at 471 >.. In order to produce a constant red, a certain amount 

 of blue had to be mixed with the extreme red of the spectrum, the latter 

 always being seen as yellow on the peripheral retina. As in the case of 

 pigment colours, the constant yellow and blue, and red and green of the 

 spectrum were found to be complementary pairs. 



Hess also investigated the peripheral limits for the colours which did 

 not change in tone, and found that both the white value and the 

 " colour value " were of influence. A green and red pigment were said 

 to have the same colour value when they had to be mixed together in 

 equal proportions in order to make grey. When the white and colour 

 values were made equal, Hess found that the peripheral limit of the 

 unchangeable red corresponded with that of the green, and of the yellow 

 with the blue. According to Hering and Hess, the outermost zone of the 

 retina is relatively insensitive to all colours, the middle zone is insensit- 

 ive to red and green, and only the central zone has normal colour vision. 

 These zones are often spoken of as zones of total colour-blindness, of red- 

 green blindness, and of normal colour vision respectively, but by this it 

 is not meant that the zones are absolutely insensitive. It would, as 

 Hess suggests, be more correct to speak of them as zones with weak 

 colour-sense, since, with sufficiently strong intensity, all colours can be 

 perceived to the extreme periphery. Hegg 2 has more recently confirmed 

 Hess's conclusion, that the borders for red and oreen coincide when 

 attention is paid to equal chromatic and white values. He found the 



1 Arch./. Ophth., 1889, Bd. xxxv. Abth. 4, S. 1. 



2 Hid., 1892, Bd. xxxviii. Abth. 3, S. 145. 



