160 COLOR-BLINDNESS IN ITS RELATION TO 



colors. He perceives, in the main, the same kind of light as the normal 

 observer, but sees a part of it in another manner. In the system according 

 to which he arranges his colors, he has fewer kinds than the normal ob- 

 server, and this is why he is obliged to classify under the same denomina- 

 tion a portion of the colors classed by the normal observer under different 

 heads. It results from this that he finds resemblances between colors or 

 confuses others that the normal observer finds different ; for instance, red 

 and green. These confusions naturally surprise and amuse the normal 

 observer, who readily imagines that it arises from very great ignorance of 

 colors, or from defective training. He ordinarily supposes that there is 

 no limit to the mistakes the color-blind might make in this respect. But 

 such is not the case ; he obeys laws quite as exact as does the normal 

 observer ; a color-blind person can no more accustom himself to seeing 

 colors as the normal observer does than the red-blind can see colors in 

 the same way that the green-blind does, or conversely. 



This theory, which is based upon experience, explains to us how the 

 color-blind see colors. But if we only base our ideas on the names 

 given to colors by the color-blind, we can be easily deceived. To judge 

 correctly of color-blindness, and the various practical questions con- 

 nected with it, it is of the highest importance to distinctly observe the 

 difference between the manner in which the color-blind person sees and 

 the manner in which he names colors. The sensation is based upon the 

 nature of the sense of color in the organization of the optic nerve from 

 birth. The name, on the contrary, is learned 5 it is conventional; it de- 

 pends upon exercise and habit. The names of colors are naturally the 

 objective expression of subjective sensations ; but, on the other hand, 

 they are regulated by the system of normal sight, and cannot conse- 

 quently agree with that of the color-blind. They can, nevertheless, be 

 learned by the latter, and even applied correctly in many cases. There 

 is connected with this fact a peculiarity of the utmost importance 

 practically to the question in point, and one that has given rise to the 

 most serious embarrassments and misunderstandings. This has been 

 and is still one of the chief causes of our erroneous ideas on the subject 

 of color-blindness existing in the masses, because it is the veil under 

 which this defect usually conceals itself from our observation in every- 

 day life, and under which, even to the last moment, it will succeed in 

 escaping discovery in cases where, as frequently happens, the methods 

 of exploration employed are indecisive or are based upon erroneous 

 principles. 



If we reflect on the condition of the color-blind, it is difficult to under- 

 stand how he can avoid being detected in his daily intercourse with 

 men endowed with normal sight. And yet experience has sufficiently 

 controverted this idea. That which we have acquired in examining en 

 masse the personjiel of a railway, for example, where it is required night 

 and day to give attention to colored signals, is singularly worthy of notice. 

 We learn by it that a number of colorblind were discovered, although 



