CIT. xxxiv. BUNS EN AND KIKCHIIOFF. 339 



fourteen years ago, Bunsen and Kirchhoff, two celebrated 

 professors of chemistry and physics at Heidelberg, discovered 

 the secret. 



These two men had been making a long set of careful 

 experiments upon all the different substances of our globe, 

 burning them and examining their vapours one by one, 

 and marking the bright lines of each upon the spectrum. 

 In doing this they did not use one prism only as Fraun- 

 hofer had done, but four (see Fig. 61), so arranged that 

 the light coming in through a slit at the beginning of the 



FIG. 61. 

 Kirchhoff s Spectroscope (Roscoe). 



tube A, was spread out more and more through each prism 

 as it passed, and fell in a spectrum on the object glass, c, of 

 the telescope B, through which they examined it. They 

 soon found that in order to mark the exact position of the 

 bright lines of each gas upon the spectrum, they wanted some 

 fixed measure, and it occurred to them that the black lines 

 of the solar spectrum, which never change, would make a 

 good scale with which to compare all the others. So they 

 arranged their spectroscope in such a manner that one- half 

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