508 REVERSION OF BRIGHT LINES. 



tively small, so that the overpowering brightness of the 

 visible portion of the spectrum renders the dark lines 

 invisible. Only larger instruments will give satisfac- 

 tory results, but another method of reversing the spec- 

 trum for which our spectroscope is available will be 

 described farther on. 



When a spirit flame coloured yellow by a little salt 

 is placed between the incandescent lime and the slit of 

 the spectroscope, a dark line will appear in the spectrum 

 of the lime light precisely where the yellow sodium 

 line is seen when the lime light is hidden from view by 

 an interposed screen. If sunlight be used instead of 

 the lime light, the sodium line will be seen exactly to 

 coincide with one of the dark lines previously seen in 

 the solar spectrum, and no new dark line will make its 

 appearance in the spectrum of the sun when the yellow 

 sodium flame is between the sunlight and the slit ; 

 the corresponding dark line in the yellow will thereby 

 only become darker and more distinct than before the 

 interposition of the sodium flame. 



It appears at first surprising that a spectrum which 

 results from a superposition of the continuous spectrum 

 of the white light given out by a solid body, which as 

 we know contains all colours, and the light of an in- 

 candescent vapour, which contains one or several colours, 

 should be exactly wanting in the one or several colours 

 given out by the glowing vapour when seen alone. 

 Thus when a lithium flame is interposed so that the 

 rays from the lime-light or from the sun are obliged to 

 pass through it, we should expect as the consequence of 

 seeing both spectra together, that the red portion of the 

 continuous spectrum of the lime or the sun would 



