148 COLOK-BLINDNESS IN ITS RELATION TO 



It belongs to theory to discover the law regulating these two factors. 

 All the qualities of light must depend upon functional diflfereuces of the 

 elements of the organ of the optic nerve. A necessary alternative is, 

 either this organ has but one kind of elements, ond then different kinds 

 of undulations of the ether induce it to act under different forms, or else 

 there are several kinds of elements or terminal organs in the retina or 

 in the brain, which always act respectively in the same way, while differ- 

 ing from each other. This latter hypothesis accords better with all we 

 know otherwise of the physiology of the nervous system, and is virtually 

 but J. Miiller's principle of the specific energies of the senses applied in 

 detail to the sense of sight. It is also on this last hyi)othesis that the 

 Young-Helmholtz theory is based. 



The following is the principle upon which this theory explains the 

 qualities of light or colors. When one kind of element alone is excited 

 or set in motion, or when all are simultaneously excited, but one in a 

 higher degree than the rest, our sensation takes hold of that element as 

 the quality of the light, as the colored light or color, and particularly the 

 cardinal or primitive color, which corresponds specifically to the excited 

 element. If there are several kinds of elements, and only two of these 

 are excited or more excited than the rest, we see the light colored, but 

 of a color which constitutes the combination of the two colors corre- 

 sponding to the excited elements. It is clear that the principle on which 

 this reasoning is based gives room for the admission of as many differ- 

 ent elements, and, consequently, primitive colors, as could be desired. 

 At all events, it results from the principle that when all the kinds of 

 elements, whether there be one or several, are excited simultaneously 

 with the same force, there is no possibility of perceiving the quality of 

 light. We then see but a light in general, in contradistinction to the 

 absence of light or darkness; in other terms, we see a colorless light, or, 

 as it is called, a tchite light, and incorrectly a white color. It follows 

 therefore that when one element is principally excited while the rest 

 are also excited, but in a less degree, the perception of the specific 

 quality of the light is feeble in proportion to the degree of excitation 

 of the other elements, since this effect of their excitation must be, in 

 short, like a mixture of colorless light, or white relatively to the color in 

 question. 



Let us now see how the Young-Helmholtz theory applies the principle 

 ■we have just explained. It recognizes three cardinal colors, red, (jrien^ 

 and violet, and consequently three kinds of corresponding elements in 

 the organ of the optic nerve; elements respectively perce^iug red, 

 green, and violet. When the perceptive element of red is excited alone, 

 or in a greater degree, we experience the sensation of red, and so on. 

 Different kinds of undulations of the ethereal medium excite, in differ- 

 ent degrees, the different elements'^ but in such a way, however, that 

 all excite in some measure each of these elements. 



Without attempting, from a scientific point of view, to explain the dif- 



