324 NINETEENTH CENTURY. IT. in. 



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 

 of the slit was lighted by the sun and the other half by the 



FIG. 52. 



Kirchhoff 's Spectroscope (Roscoe). 



flame of a gas. In this way No. 2, Plate I., would appear 

 above, and No. 3, for example, immediately below it. 



While doing this they could nit help remarking that the 

 bright yellow line of the sodium spectrum, No. 3, was exactly 

 in the same position as the black line, r>, in the solar spec- 

 trum ; l and Kirchhoff found that when he passed a faint 

 ray of sunlight through a flame of sodium (so as to make 



1 These lines are really double when seen in a powerful spectro- 

 scope, but they appear single in a small instrument. 



