154 



COLOR-BLINDNESS IN ITS RELATION TO 



to induce this green, but a feeble violet, although very sensible to the 

 normal eye, is black to the color-blind in question. 



It is plain that the violet-blind, whose primitive colors are red and green, 

 do not confuse these colors. This kind of blindness, from the experiments 

 made so far, must be very rare. For our part we have not succeeded in dis- 

 covering more than two cases agreeing quite exactly with the descrip- 

 tion given by the theory, while the first two kinds are comparatively 

 very common. In order to be abnormal it is not necessary that a sense 

 of color should completely fulfil the conditions indicated in the types 

 we have just described. We might perfectly conceive a resultant, not 

 of an absolute absence or of a complete paralysis of one kind of percept- 

 ive elements, but solely of a comparatively very low excitability, or, if 

 preferred, of a much more limited number of one kind of elements, act- 

 ing on the retina, as compared with the two other kinds. It is very easy 

 to construct curves in conformity with this idea, and not less easy to 

 arrange in this manner a continuous series of transitions and gradual 

 forms between one kind ot complete color-blindness on one side and 

 the normal chromatic sense on the other. This kind of defective vision 

 might be called incomplete color-hlindness, to distinguish it from complete, 

 as we have just characterized the three different kinds. Our experience 

 has taught us that the intermediary forms agreeing with the data given 

 above are met in large numbers in practice and of very difl'erent degrees. 

 These are the forms we designated under the common appellation of 

 incomplete color blindness, but we can according to the theory still con- 

 ceive other forms of a defective sense of color. There is one, amongst 

 others, which has at command only one of the three kinds of elements. 

 Such a sense of sight is not properly a chromatic sense. For it, there 

 exists no specific difference in light, that is to say, no color. Every kind 

 of light here acts as if on one element alone. This is why the single 

 perception of differences of intensity of light (quantity), but not of dif- 

 ferences of color (quality), is possible. This condition may then be 

 designated under the name of total color -Ijlindness. Several cases have 

 been mentioned from time to time, but we have not succeeded in find- 

 ing a single one, and it may well be questioned whether such a case has 

 actually existed. "We may also conceive that another form of a defective 

 sense of color arises from the three kinds of elements being uniformly of 

 moderate sensibility. We are able to trace the following diagram (fig. 

 5), by which the three curves simultaneously approach the abscissa, and 

 are flattened in such a manner that the vertices disappear the first. 



Fig. 5. 



