xiii.] COMMON LINES. 177 



7. Spectral Lines common to two or more Spectra. 



We now pass to difficulties of another order met with in the 

 work of comparing the lines of the different elementary bodies with 

 the Fraunhofer lines work done chiefly in the first instance by 

 Kirchhoff, Angstrom, and Thalen. Kirchhoff was not long before 

 he found that to say that each substance had a spectrum entirely 

 and specially belonging to that particular substance was not true. 

 He says, 1 " If we compare the spectra of the different metals 

 with each other, several of the bright lines appear to coincide." 

 Now Kirchhoff was working with Bunsen as his collaborateur, 

 and therefore, as we may imagine, this was not said lightly. 

 Similarly Angstrom, who was working with the assistance of the 

 Professor of Chemistry at Upsala, was driven to exactly the 

 same conclusion. He says, 2 I translate his words " Of all 

 bodies iron has certainly produced the greatest number of lines in 

 the solar spectrum. . . . Some of these seem to be common with 

 those of calcium. " Thalen carried on this inquiry, and if one 

 compares the magnificent tables which we owe to his untiring 

 skill and industry, one is perfectly astonished to find the number 

 of coincidences which he has so carefully tabulated. It might 

 be imagined naturally that these lines were due to feeble dis- 

 persion or to impurities, but there was no spectroscopic method 

 then of establishing the latter assumption. 



8. Spectral Lines vary their Intensities by Temperature. 



We now come to the discrepancies between the spectra on 

 varying the temperature which soon forced themselves upon 

 the attention of observers. 



1 Researches on the Solar Spectrum and the Spectra of the Chemical Elements, 

 Roscoe's translation, p. 10. 



- RechercJies sur le Spectre Solairc, p. 36. 



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