REFKACTIOX BY LENSES. 77 



spects, 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-blind- 

 ness (Daltonism, achromatopsia) are sometimes incapable of 

 discriminating musical tones. This was noted in numerous 

 instances by Dr. Pliny Earle. 1 Though often congenital and 

 irremediable, it is now known that color-blindness is some- 

 times produced by the excessive use of alcohol and tobacco, 

 exposure to cold and wet, etc., and is amenable to treat- 

 ment. 3 



Refraction T>y 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 supposed 

 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 the transparent bodies bounded by 

 curved surfaces, with particular reference to the action of the 

 crystalline lens. 



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

 the refraction of light, may be readily understood if we sim- 

 ply 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 the air, it is 

 again refracted, but then the deviation is from the perpen- 

 dicular of the second surface of the prism. If we imagine 

 two prisms placed together, as in Fig. 6, the ray A B will be 

 bent toward the perpendicular G B to M. As it passes from 

 the prism, it will be refracted from the perpendicular H M 

 and take the direction M I. Corresponding refraction takes 

 place in the ray !N" O falling upon the lower prism. These 

 two rays will cross each other at the point L. 



1 EARLE, On the Inability to distinguish Colours. American Journal of the 

 Medical Sciences, Philadelphia, 1845, New Series, vol. ix., p. 351. 



2 DERBY, Color-blindness, and its Acquisition through the Abuse of Alcohol 

 and Tobacco. New York Medical Journal, 1871, vol. xiii., p. 284, et seq. 



