92 LIGHT. 



which Newton divided the spectrum, not one is a pure colour. 

 The orange is produced by a predominance of the yellow and 

 red rays ; the green, by the yellow and blue rays, and the indigo 

 and violet are essentially blue, with different proportions of 

 red and yellow.* 



By placing a second prism a d c, in a reversed position, in 

 contact with the first prism, the colours disappear, and we have 

 a spot of white light, as if both prisms were absent. The three 

 coloured rays of the spectrum, therefore, produce white light 

 by their union. 



On examining the solar spectrum, Dr. Thomas Young ob- 

 served that it is crossed by several dark lines, that is, that there 

 are interruptions in the spectrum, where there is no light of any 

 colour. Fraunhofer subsequently found that the lines in the 

 spectrum of solar light were much more numerous than Dr. 

 Young had imagined, while the spectrum of artificial white 

 flames contains all the rays which are thus wanting. One of the 

 most notable is a double dark line in the yellow, which occurs 

 in the light of the sun, moon, and planets. In the light of the 

 fixed stars, Syrius and Castor, the same double line does not 

 occur ; but one conspicuous dark line in the yellow, and two in 

 the blue. The spectrum of Pollux, on the contrary, is the same 

 as that of the sun. Now a very recent discovery of Sir D. Brew- 

 ster has given these observations an entirely chemical character. 

 He has found that the whits light of ordinary flames requires 

 merely to be sent through a certain gaseous medium (nitrous 

 acid vapour) to acquire more than a thousand dark lines in its 

 spectrum. He is hence led to infer that it is the presence of 

 certain gases in the atmosphere of the sun, which occasions the 

 observed deficiencies in the solar spectrum. We may thus have 

 it yet in our power to study the nature of the combustion which 

 lights up the suns of other systems. (Report upon Optics.) 



The rays of heat are distributed very unequally throughout 

 the luminous spectrum ; most heat being found associated with 

 the red or least refrangible luminous rays, and least with the 

 violet rays. Indeed when the solar beam is decomposed by a 

 prism of a highly diathermanous material, such as rock salt, 



* Sir David Brewster, On a new analysis of the solar light, indicating three 

 primary colours, forming coincident spectra of equal length. Edinburgh I'liil, 

 Trans, vol. xii. p. 123. 



