COLOR-BLINDNESS: ITS PRACTICAL IMPORTANCE. 861 



it was first accurately described in 1794 by the physicist Dalton, who was him- 

 self red-blind. The designation color-blindness was given the condition by 

 Brewster. 



The adherents of the Young-Helmholtz theory assume the following varieties 

 of color-blindness, corresponding to paralysis of the three color-perceiving ele- 

 ments of the retina: (i) Red-blindness; (2) green-blindness; (3) violet-blind- 

 ness. In addition there is the most pronounced form total color-blindness. 

 The adherents of E. Bering's color-theory distinguish the following varieties: 



1. Complete Color-blindness (Achromatopsia). The spectrum appears achro- 

 matic, the green-yellow portion is the brightest, and the adjacent parts on either 

 side are darker. A colored painting appears like a photograph or an engraving. 

 Occasionally the different degrees of light-intensity are recognized as one shade 

 of color (for instance yellow) , which cannot be compared with any other color. 

 O. Becker and v. Hippel observed cases of unilateral, congenital complete color- 

 blindness, in which the other eye had normal color-perception. 



2. Blue-yellow Blindness. The spectrum is dichromatic, consisting only of 

 red and green; the blue-violet end of the spectrum usually is greatly shortened. 

 In pure cases, only the spectral red and green are recognized correctly (Mauthner's 

 erythrochloropia) , not however the other colors. This has been observed also 

 unilaterally. 



3. Red-green Blindness. The spectrum is dichromatic; yellow and blue are 

 recognized correctly, while violet and blue are both seen as blue. The perception 

 of red and green is absent. In this category the following types are further 

 distinguished: (a) Green-blindness or red-green blindness without shortening 

 of the spectrum (Mauthner's xanthocyanopia) , in which light green and dark 

 red are confounded. In the spectrum yellow passes directly into blue, or at most 

 a band of gray lies between the two. The maximum of brightness lies in the 

 yellow. This defect may be unilateral; it is often hereditary. (b) Red-blind- 

 ness (or red-green blindness with shortening of the spectrum; also designated 

 Daltonism}, in which light red is confused with dark green. The spectrum con- 

 sists of yellow and blue, but the yellow lies in the orange, and the red end of the 

 spectrum is colorless or even dark. The greatest illumination, as well as the 

 boundary between yellow and blue, lies more to the right. Between these two 

 forms there are transitions. According to Hering the cause of the difference 

 resides in a variation in the amount of absorption by the macula lutea of the rays 

 of short wave-length. 



4. Incomplete color-blindness, or diminished color-sense, is that condition 

 in which the acuteness of color-perception is lowered, so that colors are recog- 

 nized, for example, only in objects of considerable size or only at near range; 

 also, on addition of white they are no longer perceived as such. A certain degree 

 of this form is frequent, in so far as many are unable to distinguish between 

 greenish blue and bluish green. 



Acquired color-blindness occurs also in connection with diseases of the retina, 

 and inflammation and atrophy of the optic nerve, with beginning tabes, with 

 cerebral diseases and with intoxications (tobacco, alcohol, etc.). Green-blindness 

 appears first, and is followed shortly by red-blindness. The peripheral zone of 

 the retina suffers before the central portion. In cases of hysteria and of epilepsy 

 there may be intermittent attacks of color-blindness; and also in hypnotized 

 individuals. 



The retina may be made temporarily color-blind for a given color by intense 

 action of the color. Prolonged gazing into the dark-red, setting sun causes 

 scarlet to appear black. 



Finally, the remarkable observation of H. Cohn must be mentioned; he found 

 that color-blindness in several individuals disappeared temporarily on heating 

 the eyeball. Holmgren found that 2.7 per cent, of persons examined were color- 

 blind, most of them being red-blind and green-blind; only a few were violet-blind. 



The examination of the power of color-perception in the normal retina, best 

 made with the Aubert-Forster perimeter, has revealed the surprising fact that 

 complete color-perception is present only in the center of the visual field. Around 

 this lies a middle zone, in which only blue and yellow are perceived, and in which 

 there is, therefore, red-blindness. Beyond this zone there is, finally, a peripheral 

 girdle, in which there is complete color-blindness. The red-blind individual 

 is distinguished, therefore, from the normal by an absence of the central area 

 of the normal visual field, which is included in the middle zone. The visual 

 field of the green-blind individual is distinguished from that of the normal by 



