156 THE SOLAR SPECTRUM. SECT. XIX. 



variety of colored media, Sir David Brewster, so justly 

 celebrated for his optical discoveries, has proved that 

 the solar spectrum consists of three primary colors, red, 

 yellow, and blue, each of which exists throughout its 

 whole extent, but with different degrees of intensity in 

 different parts ; and that the superposition of these three 

 produces all the seven hues according as each primary 

 color is an excess or defect. Since a certain portion of 

 red, yellow, and blue rays constitute white light, the 

 color of any point of the spectrum may be considered 

 as consisting of the predominating color at that point 

 mixed with white light. Consequently, by absorbing 

 the excess of any color at any point of the spectrum 

 above what is necessary to form white light, such white 

 light will appear at that point as never mortal eye 

 looked upon before this experiment, since it possesses 

 the remarkable property of remaining the same after 

 any number of refractions, and of being capable of de- 

 composition by absorption alone. 



In addition to the seven colors of the Newtonian spec- 

 trum, Sir John Herschel has discovered a set of very 

 dark red rays beyond the red extremity of the spec- 

 trum, which can only be seen when the eye is defended 

 from the glare of the other colors by a dark blue cobalt 

 glass. He has also found that beyond the extreme 

 violet there are visible rays of a lavender gray color, 

 which may be seen by throwing the spectrum on a 

 sheet of paper moistened by the carbonate of soda. 

 The illuminating power of the different rays of the spec- 

 trum varies with the color. The most intense light is 

 in the mean yellow ray. 



When the prism is very perfect and the sunbeam 

 small, so that the spectrum may be received on a sheet 

 of white paper in its utmost state of purity, it presents 

 the appearance of a riband shaded with all the prismatic 

 colors, having its breadth irregularly striped or subdi- 

 vided by an indefinite number of dark, and sometimes 

 black, lines. The greater number of these rayless lines 

 are so extremely narrow that it is impossible to see 

 them in ordinary circumstances. The best method is 

 to receive the spectrum on the object glass of a tele- 

 scope, so as to magnify them sufficiently to render them 



