REFRACTION IN THE EYE. 737 



It is a curious fact that the mixture of different colors in certain proportions will 

 result in white. Two colors, which, when mixed, result in white, are called complemen- 

 tary. The following colors of the spectrum bear such a relation to each other : 



Red and greenish -blue. 



Orange and cyanogen-blue. 



Yellow and indigo-blue. 



Greenish-yellow and violet. 



The fact that impressions made upon the retina persist for an appreciable length of 

 time enables us to illustrate the law of complementary colors. If a disk, presenting 

 divisions with two complementary colors, be made to revolve so rapidly that the impres- 

 sions made by the two colors are blended, the resulting color is white. 



It is almost useless, with our present knowledge, to speculate with regard to the prob- 

 able mechanism of the appreciation of colors in vision. The facts just stated are suffi- 

 ciently clear, showing that the number of ethereal vibrations is different for different 

 colors ; but it is by no means determined that differences in the amplitude of the vibra- 

 tions are in direct relation with the arrangement of the disks of the rods and cones in 

 different portions of the retina, a theory lately proposed by Zenker. The curious phe- 

 nomena of color-blindness depend upon an abnormal condition of the visual apparatus. 

 Persons possessing this peculiarity called sometimes Daltonism, after the celebrated 

 English chemist, who described this infirmity as it existed in his own person although 

 vision may be normal in other respects, cannot distinguish certain colors, will mistake red 

 for green, etc., and some can only distinguish black and white. It is a curious fact, also, 

 that persons affected with color-blindness (Daltonism, or achromatopsia) are sometimes 

 incapable of distinguishing different musical tones. Although often congenital and irre- 

 mediable, it is now known that color-blindness is sometimes produced by the excessive 

 use of alcohol and tobacco, exposure to cold and wet, etc., and is amenable to treatment. 



Refraction ly Lenses. A ray of light is an imaginary pencil, so small as to present 

 but a single line ; and the light admitted to the interior of the eye by the pupil is sup- 

 posed to consist of an infinite number of such rays. In studying the physiology of 

 vision, it is important to recognize the laws of refraction of rays by transparent bodies 

 bounded by curved surfaces, with particular reference to the action of the crystalline lens. 



FIG. 251. Refraction by prisms. 



The action of a double-convex lens, like the crystalline, in the refraction of light, 

 may be readily understood if we simply apply the well-known laws of refraction by 

 prisms. A ray of light falling upon the side of a prism at an angle is deviated toward 

 a line perpendicular to the surface of the prism. As the ray passes from the prism to 



