io 9 2 VISION. 



In 1879, v. Kries and Kuster l determined the amounts of spectral red 

 and blue necessary to match a certain blue-green, and found that the 

 cases examined fell into two sharply-defined groups. Eecently, v. Kries 2 

 has determined in twenty cases the amount of lithium-red necessary to 

 match sodium-yellow, and has found two distinct groups, the members of 

 each differing very little from one another but very greatly from those 

 of the other group, i.e. to ten parts of the yellow the members of one 

 group required 36'3 to 40 parts of red light, while those of the other 

 varied from 196 to 225. 



Donders 3 compared the relative intensities of lithium-red, sodium- 

 yellow, and thallium-green in twenty cases of colour-blindness, and 

 found that they fell into two widely separated groups, the relative 

 intensities of the three lights showing a close correspondence in 

 photerythrous and normal vision. 



In two scoterythrous and two photerythrous cases, v. Kries 4 obtained 

 matches of a mixture of blue and red light with a number of inter- 

 mediate homogeneous lights. Eepresenting the amounts of red and blue 

 light respectively by curves, v. Kries found that the red curves of the two 

 scoterythrous cases corresponded closely with each other, and similarly 

 those of the two photerythrous cases ; but the red curves of the two groups 

 differed greatly from one another, the former having its maximum at 

 571 X, while that of the latter was at 603 X. The blue curves of all four, 

 on the other hand, showed a much closer correspondence with each other. 



According to Hering and Hess, there is an almost exact corre- 

 spondence between the vision of total colour-blindness and that of the 

 extreme periphery of the normal retina, and also between red-green 

 blindness and the red-green blind zone of the normal eye. Hess found 

 that colour matches good for his red-green blind zone were good for two 

 red-green blind cases of the photerythrous type. 



v. Kries 5 has lately brought forward evidence of a very different 

 relation between the two. He finds that the curve of brightness of the 

 spectrum at the extreme periphery of the normal eye corresponds closely 

 with that for the paracentral region, the maximum brightness being 

 between 608 X and 589 X, while the curve differs enormously from that 

 of congenital total colour-blindness. He finds that the periphery values 

 of the normal eye differ very greatly from those of the scoterythrous 

 group of red-green blindness, the curve of the latter having its maximum 

 at 558 X. The values of a photerythrous case agree much more closely 

 with the normal, but show a greater deviation than is found between 

 different cases of normal vision. Polimanti 6 has also found that the 

 values obtained by the nicker method for lights between 664 X and 

 526 X differ in the normal and colour-blind eyes. This was especially 

 marked in a scoterythrous case, in whom the maximal value (i.e. bright- 

 ness) was at 565 X by the flicker method, while the maximum for a 

 normal eye was at 589 X. A photerythrous case agreed much more 

 closely with the normal, the maximum being in this case, however, 

 displaced somewhat towards the red end of the spectrum. 



"Weak colour- vision and individual variations. In addition to 



1 Arch.f. PhysioL, Leipzig, 1879, S. 513. 



2 Ztschr.f. Psychol. u. PhysioL d.Sinnesorg,, Hamburg u. Leipzig, 1897, Bd. xiii. S. 259. 



3 Arch.f. PhysioL, Leipzig, 1884, S. 528, ' 4 Loc. ciL, S. 274. 



5 Ztschr. /. Psychol. u. PhysioL d. Sinnesorg., Hamburg u. Leipzig, 1897, Bd. xv. S. 247. 



6 Ibid., 1899, Bd. xix. S. 263. 



