REFRACTION OF LIGHT. 433 



and red and green, are, in the same manner, complementary colors. 

 Such complementary colors may be primary or secondary. If decom- 

 position by absorption takes place of all the white light reflected from 

 the surface on which it falls, the color of the object is intense ; but if 

 part of the white light be not decomposed, the reflected color is diluted 

 by it, and is much less intense. Translucent bodies may decompose 

 white light, in this manner, both reflecting and transmitting various 

 colors. Those transparent bodies or media, which, besides transmit- 

 ting light, cause its decomposition by absorption, are both colored and 

 transparent. It has already been mentioned, that absorption of some 

 of the rays always takes place when light passes through a body, 

 however great its transparency ; it is thus that the phenomena of aerial 

 perspective are produced. The rays given off by artificial lights pre- 

 sent many varieties in color. Some are monochromatic, giving out but 

 one color. Their calorific or heating power, and their chemical action, 

 are all exceedingly different. All such artificial rays are less powerful 

 than the solar rays. 



Rays of light, as already stated, travel in straight lines ; and so long 

 as they pass through a medium of uniform density, and also when they 

 pass from a rarer into a denser, or from a denser into a rarer medium, 

 as from air into glass (Diagram H, P), or from glass into air, /?, in a 

 direction perpendicular to the surfaces of the media, they continue to 

 move on in straight lines, P p. But when rays pass obliquely, 0, from 

 one medium into another of different density, they are bent out of their 

 straight course, undergoing what is called refraction. When the rays 



Diagram H. 



H 



7 



Diagram H. Showing simple refraction of light. G, G, piece of glass. P. p, perpendicular ray of light, 

 passing from air into glass, and from glass into air, without change of direction. 0, oblique ray, bent, on 

 passing from air into glass, o, and again bent, o', on passing from glass into air. 



pass from a rarer into a denser medium, o, 0, they are bent towards a 

 line perpendicular to the surface of the media, at the point of incidence; 

 but when rays pass from a denser into a rarer medium, 0, o f , they are 

 bent from that perpendicular. The incident and the refracted rays 

 always lie in the same plane. The refractive powers of different 

 media present considerable differences ; thus the refractive index, or 

 relative refractive value of air, vacuum being taken as 1, is 1.003, of 

 water 1.33, of flint glass 1.642, of the diamond 2.755. The amount 

 of refraction increases with the obliquity of the incident rays; this 

 increase follows " the law of the sines." The refractive power of a 

 medium increases generally with its density, and with the retardation 



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