SCIENCE. 



LN. S, Vol. VIII. No. 193. 



It is evident that a simple absence of one 

 of the fundamental sensations is not enough 

 to account for the facts. 



But more than the ordinary color-blind- 

 ness comes into the question. It was dis- 

 covered that the normal eye is color-blind 

 by indirect vision, the condition gradually 

 increasing from the central portion out- 

 ward, and culminating in the periphery of 

 the retina in absolute insensibility to color. 

 The sensation of white cannot in this case 

 be produced by a mixture of three color- 

 sensations, because color- sensation does not 

 exist. 



In 1777 was described, in the Philosoph- 

 ical Transactions of the Eoyal Society, the 

 well-known case of the shoemaker Harris, 

 apparently the first recorded case of com- 

 plete color-blindness. In 1871 Bonders 

 made a careful study of another case. So 

 rare is this defect, however, that so assidu- 

 ous an observer as Holmgren in 1877 bad 

 never seen it and doubted its existence, and 

 in 1889 Helmholtz simply passes it by, say- 

 ing that the eye so affected is always in 

 other ways ailing and sickly, the implica- 

 tion being that the effect was so far from 

 the ordinary and so pathological in char- 

 acter that it need not be considered. But 

 by 1892 a sufficient number of such cases 

 had been examined to establish the existence 

 of this as a distinct type of color-vision, and 

 one which must be accounted for. 



A third and even more striking depar- 

 ture from the simple conditions of the 

 original theory was found in the remark- 

 able change in the character of color-sensa- 

 tions in faint light, the spectrum becoming 

 simply a colorless strip of graduated bright- 

 ness. 



We have then three diiferent types of 

 varying color sensibility : 



1. In the eyes of diiferent persons, be- 

 ginning with the normal-eyed, passing 

 through the variation studied by Rayleigh 

 and Bonders, which while recognizing all 



colors, yet in the character of the sensations 

 makes a step toward green-blindness, then 

 the two well-marked divisions of the color- 

 blind, red- and green-blind, and finally th& 

 eye which perceives no color. These dif- 

 ferent classes are well marked, and few 

 intermediate forms are found. 



2. In the same eye, under differing de- 

 grees of brightness. As the spectrum' 

 gradually diminishes in brightness the 

 colors change. Eed tends toward yellow ,^ 

 yellow toward green, green becomes bluish. 

 All the colors vanish by degrees, red disap- 

 pearing first, and the spectrum, still visible 

 through the greater part of its length, ap- 

 pears without color. Here the intermediate 

 stage is a sort of red-blindness, but the final 

 result is the same for all eyes, that is, the dis- 

 tribution of brightness for the normal and 

 the color-blind eye appears to be precisely 

 the same, a circumstance, as we shall see, 

 of much importance. 



3. In a single eye, passing from the cen- 

 ter outward. The fovea and the zone sur- 

 rounding it are sensitive to all colors. 

 Outside of this the eye becomes gradually 

 insensitive to color, the sensation of red and 

 green disappearing first, afterwards yellow, 

 and finally blue. The outer part of the 

 retina is insensitive to all color, but its con- 

 dition may be quite different from that of 

 the eye of a totally color-blind person, or a 

 normal eye in faint light.* 



In attempting to account for these varied 

 phenomena the Helmholtz theory loses its 

 striking simplicity. It is no longer possible 

 to explain color-blindness by the absence of 

 one or more sets of sensitive fibres, or the 

 white seen by a totally color-blind retina, 

 as compounded from the three fundamental 

 sensations, in any ordinary way. 



Helmholtz, in 1860, made the suggestion, 

 which was afterward amplified by Fick and 

 Konig, of a possible changeability in the 



* von Kries, Centralblatt f. Physiologie, X., pp. 745- 

 749, 1896. 



