236 THE CHEMISTRY OF THE SUN. [CH. xvn. 



let us say, of coincident lines in the spectrum of iron and cobalt, 

 the common line was due to an impurity either of iron in the 

 cobalt or of cobalt in the iron. Most spectroscopic workers 

 were of the true faith in this matter ; they accepted the dicta 

 of the chemist, and not only was the work which had shown 

 how the phenomena observed might be thus explained received 

 with favour, but no one, so far as I know, inquired whether 

 there was any other " might be " in the matter. 



The other set of lines was as different as possible. Of them 

 there was, on the impurity hypothesis, no possible explanation 

 forthcoming without changing ground. In fact, the separation 

 of the coincidences into two classes was brought about by this 

 very circumstance, since all the coincidences which, in accord- 

 ance with a general law established for a constant temperature 

 some years before, could be attributed to impurity had, as a 

 matter of fact, been eliminated from the maps at a prior stage 

 of the investigation. Further, be it noted that all the photo- 

 graphs represented the work of similar temperatures, for they 

 were all taken with electric arcs, for the production of which 

 the same number of Grove's cells was used in all cases. 



It is amusing to go back to the old observation books, and to 

 see with what pertinacity for the first two years we stuck to the 

 possibility that the solar line or the metallic line we were dealing 

 with was a double line, and then, after we had to give that idea 

 up, as the coincidences became of three- four- five- and six-fold 

 complexity, we came to the conclusion that we were dealing 

 with a common impurity. That of course was a point we could 

 not settle until we had gone through all the chemical elements 

 which were known to us, and it was the discussion of the 

 spectra of so many substances which took up so much time. 



