io 9 4 VISION. 



sented a neutral region beyond the point where colour could be per- 

 ceived. The most saturated green was seen at 532 X, and the place of 

 maximum brightness was more towards the red end, between 558 X and 

 565 X. The case recorded by Donders was very similar, the spectrum being 

 shortened at each end, and the neutral band in the yellow was even broader 

 than in the other case, extending to one-third of the length of the spectrum. 



Unilateral cases.— The subjective nature of the sensations experi- 

 enced by the congenitally colour-blind can, in ordinary cases, only be 

 a matter of speculation. Individuals, who see the spectrum from the 

 red end to the neutral band in only one colour, may nevertheless, under 

 many circumstances, distinguish red from yellow and yellow from green, 

 but accurate observers recognise that they are enabled to do this owing 

 to differences of saturation. Eed is especially distinguished as a dark 

 colour with a very high degree of saturation. Most observers, such 

 as Dr. Pole, 1 who has given a classical account of his own condition, 

 believe that the two colours they experience correspond to the yellow 

 and blue of the normal eye. A few cases have been recorded in which 

 the congenital defect was unilateral, and the sensations of the defective 

 could be compared with those of the normal eye. One such case 

 investigated by v. Hippel 2 and Holmgren, 3 was of the ordinary type, in 

 which red and green are confused. There is some doubt as to the sub- 

 group to which this subject belonged ; according to Holmgren, he was a 

 typical example of the group in which the spectrum is shortened at the red 

 end ; while, according to v. Hippel, 4 he could distinctly see as far as the 

 rubidium line y, which he called faint yellow. Both agree that the two 

 colours of this case corresponded with the yellow and blue of the normal 

 eye, the yellow corresponding to the colour of I), and the blue to the 

 indium or ctesium line of the normal eye. 



In another unilateral case recorded by Holmgren, blue and yellow 

 were confused ; the neutral band was just on the green side of D ; the 

 two colours compared with those of the normal eye were red and green, 

 the red corresponding to carmine and the green inclining to blue. There 

 was no sensation corresponding to the yellow of the normal eye. 



In a third unilateral case recorded by Becker, 5 the condition was one of 

 total or almost total colour-blindness. The defect was said to have existed 

 from an early age. All colours were confused, with the exception of brown, 

 which seems to have been correctly recognised. The case differed in several 

 respects from typical total colour-blindness. There was no shortening 

 of the red end, and slight shortening at the violet end. The point of 

 maximum brightness was almost exactly at the sodium line. Visual 

 acuity was normal, and no mention is made of photophobia or nystagmus. 



In other unilateral cases which have been described, the defect was 

 probably acquired. 



Acquired colour-blindness.— As regards the subjective nature of 

 the sensations experienced, it might be expected that acquired cases 

 would be more useful than congenital, since the individual would 

 have the memory of his normal vision as a standard. These cases, how- 

 ever, have been much less fully studied. Acquired colour-blindness 



1 Phil. Trans., London, 1859, vol. cxlix. p. 323. 



2 Arch. f. Ophth., 1880, Rd. xxvi. Abth. 2, S. 176; and 1881, Bd. xxvii. Abth. 3, 

 S. 47. 



3 Centralhl.f. d. mod. TVissensch., Berlin, 1880, S. 898 and 913. 



4 Arch.f. Ophth., 1879, Bd. xxv. Abth. 2, S. 205. 



5 Centralbl. f. prakt. Augcrih., Leipzig, 1888, S. 353. 



