REFRACTION IN THE EYE. 693 



that it is compound ; and by synthesis, the colored rays may be brought to- 

 gether, producing white light. Colors may be obtained by decomposition of 

 light by transparent bodies, the different colored rays being refracted, or bent, 

 by a prism, at different angles. It is not in this way, however, that the colors 

 of different objects are produced. Certain objects have the property of re- 

 flecting the rays of light. A perfectly smooth, polished surface, like a mir- 

 ror, may reflect all of the rays ; and the object then has no color, only the re- 

 flected light being appreciated by the eye. Certain other objects do not reflect 

 all of the rays of light, some of them being lost to view, or absorbed. When 

 an object absorbs all of the rays, it has no color and is called black. When 

 an object absorbs the rays equally and reflects a portion of these rays without 

 decomposition, it is gray or white. There are many objects, however, that 

 decompose white light, absorbing certain rays of the spectrum and reflecting 

 others. The rays not absorbed, but returned to the eye by reflection, give 

 color to the object. Thus, if an object absorb all of the rays of the spectrum 

 except the red, the red rays strike the eye, and the color of the object is red. 

 So it is with objects of different shades, the colors of which are given simply 

 by the unabsorbed rays. 



A mixture of different colors in certain proportions will result in white. 

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

 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 affords an illustration of the law of complementary colors. If 

 a disk, presenting divisions with two complementary colors, be made to 

 revolve so rapidly that the impressions made by the two colors are blended, 

 the resulting color is white. 



Refraction by 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 re- 

 fraction of rays by transparent bodies bounded by curved surfaces, with par- 

 ticular reference to the action of the crystalline lens. 



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

 of light, may readily be understood by a simple application of 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 the de- 

 viation is then from the perpendicular of the second surface of the prism. 

 In passing through a prism, therefore, the pencil of light is bent, or refracted, 

 toward the base. 



A circle is equivalent to a polygon with an infinite number of sides. A 

 regular, double-convex lens is a transparent body bounded by segments of a 

 sphere. Theoretically a double-convex lens may be assumed to be composed 

 of an infinite number of sections of prisms .(Fig. 254, 1.), or to make the com- 



