REVERSION OF BRIGHT LINES. 507 



spirit lamp which is fed by a mixture of spirit and 

 ether, and directing a current of pure oxygen upon it, 

 the lime is brought to a state of white heat, and emits 

 a brilliant light, called 4 D^ummond's lime light.' The 

 spectrum of this light is continuous. If the flame of a 

 common spirit lamp is interposed between the lime 

 light and the slit of the spectroscope so that the rays 

 of the lime light have to pass through the spirit flame 

 before reaching the slit, no sensible change will be 

 perceived in the spectrum of the lime light; but if 

 some chloride of lithium be placed upon the wick 

 of the lamp so that the flame appears red, a dark 

 line will be seen in the red portion of the formerly 

 continuous lime-light spectrum. If now an opaque 

 screen is placed between the lime light and the coloured 

 spirit flame, so that the light of the spirit flame alone 

 falls upon the slit, the well-known red lithium line will 

 make its appearance exactly where the dark line was 

 seen in the spectrum of the lime light when it was 

 viewed through the spirit flame coloured by lithium. 



This i reversion ' of lines of the spectrum, that is, 

 the appearance of dark lines in the exact place of bright 

 luminous ones, may be observed by using the light of 

 the sun instead of the lime light. There is no dark line 

 in the solar spectrum corresponding to the position of 

 the lithium line, but if a spirit flame coloured by lithium 

 is placed in front of the slit of the spectroscope, so that 

 the sun's rays have to pass through it a dark line will at 

 once appear in the red portion of the solar spectrum. 



This method of reversion is not practicable with, 

 a small spectroscope, like that used for the preceding 

 experiments ; the dispersion in that case is compara- 



