REVERSION OF BRIGHT LINES. 511 



spirit flame coloured strongly yellow by common salt 

 between the eye and our spectroscope, as in fig. 281. 



Before the spirit flame is interposed, the bright spec- 

 trum of the incandescent carbon particles in the paraffin 

 flame is seen to be continuous. The spirit flame is then 

 placed as near as possible to the aperture of the spectro- 

 scope, and the eye brought as near to the flame as can 

 be done without inconvenience from the heat radiated 

 by the flame ; in such close vicinity the flame is not 

 distinctly seen, but sheds a yellow tint over everything 

 seen through it, and therefore also over the spectrum pro- 

 duced by the apparatus. The spectrum is still seen, which 

 proves that the vapour is transparent for the various 

 colours, although their purity is naturally somewhat 

 impaired by the admixture of yellow light ; but in the 

 yellow part of the spectrum there now appears a dark 

 line in the exact place which corresponds to the position 

 of the yellow sodium line ; this yellow colour is therefore 

 not allowed to pass through the sodium vapour, it is 

 absorbed by it ; not completely, but sufficiently so to 

 render that part of the spectrum darker than the 

 remainder, or even nearly black by contrast. 



The slit should be narrow, not more than 0*2 or O mm- 3, and 

 the paraffin flame about 10 cm behind it. The spirit flame must be 

 strongly yellow if the absorption line is to be quite dark. By putting 

 salt on the wick and rubbing it well in with the fingers, the flame 

 will be coloured strongly yellow, but only for a few moments after 

 being lighted ; it is therefore better to mix the spirit of wine, before 

 filling the lamp, with about T V*h of its volume of water, throw in half 

 a teaspoonful of salt, and shake the liquid strongly, so as to dissolve 

 as much of the salt as possible. The flame of this alcoholic solu- 

 tion of salt will produce permanent absorption, especially if the 

 flame is from time to time blown out, and the wick rubbed between 

 the fingers. The dark line can only be well seen if the position of 





