XI.] 



USE OF THE LENS. 



141 



distinct spectra. We find that the interior of the flame, that is 

 the blue part (best observed at the bottom), gives us one spec- 

 trum, the white part gives us another, while on the outside, so 

 faint as to be almost invisible to the eye, there is a region which 

 gives us a perfectly distinct spectrum consisting of a line in the 



FIG. 60. Showing arrangement of lens with large spectroscope. A, collimator ; 

 B, observing- telescope ; c, spark ; D, lens. 



yellow. In this way there is no difficulty whatever in deter- 

 mining the co-existence of three light- sources, each with its 

 proper spectrum, in the light of a common candle or lamp. 



What a pity that Wollaston did not use such a lens in 1804 ! 



We see in a moment that much the same condition of affairs 

 will be brought about if, instead of using a candle, we use an 



