THE MACULA LUTEA AND FOVEA CENTRALIS 83 



Number of Angular Distances from Centre within which the Colour 



Observer of a Spectral Light 30" Diameter was recognised 



Red Light (X 6563) Green Light (X 51 69) 



1 Within 6 or 7 Within 1| or 2 



2 7i 8J H 21 



3 9 10 2 3 



4 .., 6 ,,7 2 ,,2J 



5 no DO oo oo 



., / ,, O t ,, O 



6 8 ,,y 2 ,,24 



7 ,. 8 ,,9 2 ,,3" 



8 8i ,,9i 2 ,,3 



9 6 ,,7J 2 ,,24 

 10 8 ,,9 2 ,,2J 



' It may be added that with the red spot of light any recognition of 

 light immediately beyond the sensitive limit given above was very 

 doubtful or non-existent, although with practice a dull grey spot can 

 be perceived for a short distance. On the other hand, with the green 

 spot of light there was, from 3 outwards, most definite recognition of 

 light over a very large area, the appearance being a white or bright 

 dazzle. This was also present with blue light, but the hue was unrecog- 

 nisable as regards both blue and green. Yellow light also gave the 

 same bright dazzle ; it was often blended with a reddish sensation if it 

 lay from the centre about 3 or 4. 



' The recognition of small areas of red or green by the dark-adapted 

 eye is, as regards colour, thus only possible when these are focussed near 

 the fovea, i.e., lie near or at the centre of the field of vision. This is 

 particularly the case with the peripheral vision of green ; presumably 

 these rays, by exciting rods, evoke a sensation of white, which has a 

 dazzling effect and masks the true colour. Such white dazzling sensa- 

 tion is still present, but to a less degree, as the green light approaches 

 the centre of the field of vision, but it ceases when this light is 1 to 2 

 from the centre. According to v. Kries the explanation is the differential 

 stimulation of both rods and cones, that of the rods giving rise to the 

 dazzling white sensation in addition to the colour sensation produced 

 by the stimulation of the cones." 



Although the fovea is night-blind compared with the periphery it 

 is capable of some degree of dark adaptation. It is necessary to have 

 very strong preliminary light adaptation, as from looking at the clear 

 sky, in order to demonstrate it (Nagel and Schafer 1 ). Tschermak 2 

 also obtained foveal dark adaptation and thought that it was slower 



1 Ztsch.f. Psi/chol. u. Physiol. d. SimiKSorg. xxxiv. 27 1, 1904. 



2 Arch. f. d. ges. Physiol. LXX. 297, 1898. 



62 



