EXPERIMENTS ON LIGHT-SPECTRA. 493 



or lithium chloride be introduced into the hydrogen 

 flame, the image of the red flame as seen through the 

 prism will appear as distinct as the sodium flame, but 

 its deviation will be less; that is, it will appear not so 

 far to the left as the sodium flame. Lithium compounds 

 purchased of the dealers contain generally slight ad- 

 mixtures of sodium compounds, and if such a com- 

 pound, or a mixture of a lithium salt and a sodium 

 salt specially prepared for the purpose, be introduced 

 into the flame, the coloration when viewed with the 

 naked eye will not appear so beautifully red as when 

 the lithium salt is free from sodium. But if such a flame 

 be observed through the prism, two distinct images of -it 

 will be seen, one by the side of the other, one having 

 the red colour of the pure lithium flame, the other 

 the yellow of the pure sodium flame. If a mixture is 

 employed which contains more of the salt of sodium 

 and less of that of lithium, the flame will appear to the 

 eye without prism simply yellow, but viewed through 

 the prism two distinct images will still appear, of which 

 the red one, however, disappears sooner than the yellow, 

 because the lithium compounds are more volatile than 

 those of sodium, and the small quantity of the former is 

 thus very soon volatilised. 



The eye is incapable of separating the two distinct 

 images which in the above experiment are due to each 

 kind of light emitted by the same flame ; and whenever 

 light is compounded in this manner, the eye sees a 

 colour which is in reality a mixture of those present in 

 the source of light. But the prism separates each kind 

 of light from the other hy refracting one kind more 

 than the other, and produces thus as many distinct 



