150 COLOK-BLINDNESS IN ITS EELATION TO 



perceptive elements of green and violet, but feebly those of red. Hence, 

 the sensation of blue, which is a combination of violet and green. Violet 

 light powerfully affects the perceptive elements of violet and feebly the 

 two others. Hence, the sensation of violet. 



These curves enable us to explain easily the colors of the spectrum by 

 the theory. We find in what proportion each one of the three fundamen- 

 tal perceptions enters into it by measuring the vertical distance from their 

 place on the horizontal line (abscissa) to the corresponding points of each 

 of the three curves. It is then seen that there is no color of the spectrum 

 into which but one primitive color enters exclusively. The two others also 

 furnish their contingent. No one is therefore perfectly "saturated." They 

 are more or less spread with white, and green is the least sensibly "sat- 

 urated," or more whitish. The curves also show us that yellow and blue 

 are at the same time whitish colors, and the most intensely luminous in 

 the whole spectrum. When we again add that a color "saturated" in 

 almost the same degree as the other spectral colors, proceeds from the 

 homogeneous combination of red and violet, that is to say, purple, and 

 its whitish shade, pink, we shall have said all, theoretically, that we 

 have to say in relation to the normal chromatic sense. 



To explain the abnormal sense of colors by the theory of the normal, we 

 can, in advance, conceive various possibilities. Let us suppose that one 

 of the three fundamental perceptions is wanting, or that one of the prim- 

 itive colors is absent; it is clear that the whole chromatic system will be 

 upset. It is evident, therefore, that this system must be completely dif- 

 ferent, according to the absence of one or the other of the three primi- 

 tive colors. It is virtually just in this way that it has been attempted 

 to explain cases of a strongly marked defect in the chromatic sense, or 

 genuine types of blindness to color, found in real life; The term color- 

 hlindness has been justified by this, as it indicates in each case a genuine 

 blindness to one of the cardinal colors. In this way, therefore, we dis- 

 tinguish, according to the kind of element wanting, three classes of 

 blindness : 



1st. Eed-blindness. 



iid. Green-bliudness. 



3d. Violet-blindness. 



We shall see that the Young-Helmholtz theory, as we have explained 

 it far from being contradicted, as has been recently claimed, by the 

 phenomenon of color-blindness, finds in it, on the contrary, a support, 

 and this theory most certainly furnishes the best guide for attaining 

 the practical end in view for which we intend to use it. Let us, in the 

 first instance, cast a rapid glance over the different kinds of typical and 

 complete blindness to colors, as their features are presented by the 

 theory. This sketch will be singularly facilitated by the use of the same 

 kind of curves employed in illustrating the normal sense of colors. 



1. According to the theory, blindness to red is due to the absence or 

 paralysis of the organs perceiving red (fig. U). Ked-blindness has then 



