ADVANCE OF ASTRONOMY. 213 



that two adjacent yellow lines in the spectrum of a 

 candle flame (now known to be due to sodium) coin- 

 cided with a pair of dark lines in the solar spectrum. 

 Similarly Brewster showed that the potassium lines 

 coincide with other Fraunhofer lines. 



In 1822 Sir John Herschel noted the bright lines 

 of flames in which certain metallic salts are burnt, 

 and in 1825, along with Talbot, he suggested the 

 importance of using the spectroscope to detect the 

 presence of minute quantities of certain substances 

 in minerals. In 1826 Talbot almost reached the fun- 

 damental conclusion that the presence of a certain 

 line in the spectrum tells unerringly that a certain 

 substance is glowing in the fire of the luminous body. 

 Brewster followed on the same track, and William 

 Swan noted the delicacy of the spectroscopic test in 

 detecting the presence of various substances, such as 

 common salt. 



As we have already hinted, gaps or dark lines in 

 the solar spectrum mean that rays of a certain re- 

 frangibility (which depends upon wave-length) are 

 absent. It is plain that they may be absent from 

 the start or simply because they are absorbed in 

 passing through the earth's atmosphere. Thus it 

 was an important step when, in 1832, Sir David 

 Brewster noted that some of the dark lines which 

 Fraunhofer had mapped out on the solar spectrum, 

 were intensified when the sun was near the horizon, 

 that is to say when its rays have a longer path through 

 the earth's atmosphere and are therefore more liable 

 to absorption. Gaps thus due to absorption by the 

 earth's atmosphere are called " telluric lines." 



The coincidence noted by Fraunhofer between 

 two yellow lines on the sodium spectrum and a pair 

 of dark (D) lines in the solar spectrum, was carefully 



