224 THE CHEMISTRY OF THE SUN. [CHAP. 



Some spectra are full of doublets : sodium and potassium, as 

 ordinarily mapped, may be said indeed to consist exclusively of 

 doublets ; others, again, are full of triplets, the wider member being 

 sometimes on the more, 'sometimes on the less, refrangible side. 

 Doublets and triplets, as a rule, reverse themselves more freely 

 than the irregular lines in the same spectrum which particular 

 doublet or triplet will reverse depending upon the temperature- 

 The more one studies these photographic spectra in detail, and 

 especially under varying conditions of temperature which enable 

 us to observe the reversal now of this set of lines, now of that, the 

 more complex becomes the possible origin of the phenomena 

 thus permanently recorded. 



5. The Elimination of Impurities. 



The purification of the spectrum of each substance was con- 

 ducted as follows : The spectrum of the element, as stated on 

 page 215, was first confronted with the spectra of the substances 

 most likely to be present as impurities. This was most con- 

 veniently done by photographing the spectra on the same plate 

 one above the other, so that common lines were continuous. 



We had, for instance, a great number of photographs of iron, 

 to take a case. The spectrum of iron was then photographed 

 with that of each of the other metallic elements. The question 

 then was : given these photographs bristling with impurities 

 and if there were no impurities present we should not know 

 that our photograph was a good one, because the continuous 

 lines showed there was no shift how are we to produce a 

 map of the spectrum of one substance which shall be absolutely 

 purified with regard to the rest ? 



The diagram (Fig. 85) will show how the process of elimina- 

 tion which was now rendered possible was carried on. We 

 have there three hypothetical spectra, with their long and short 

 lines. We compare A with B, and find that in the photograph 



