[ 33 ] 



IV. On Relations between the Lines of various Spectra, with 

 Special Reference to those of Cadmium and Zinc, and a 

 Redetermination of their Wave-lengths. By Joseph 

 Sweetman Ames, Assistant in Physics, Johns Hopkins 

 University.* 



IN this paper I shall give a sketch, as complete as possible, 

 of the many theories which have been advanced concern- 

 ing spectral relations, with a short statement as to their agree- 

 ment with the most recent investigations ; and then I wish to 

 call attention to a few new relations suggested by my own 

 studies. 



Relations between Different Spectra. 



Alex. Mitscherlich | was the first, apparently, to seek 

 numerical relations between the spectra of different substances. 

 Having proved, independently of Roscoe, that the spectra of 

 compounds were different from those of the elements them- 

 selves, he examined and compared the spectra of the haloid 

 salts of barium, calcium, and strontium. He selected certain 

 lines in each as most prominent, and found in the case of the 

 salts of barium that the distance apart of these lines was 

 directly proportional to the atomic weight of the salts them- 

 selves, while in the calcium and strontium salts the proportion- 

 ality was an inverse one. He could find no relations in the 

 spectra of the elements. Those found in the compounds were 

 chance ones; for, using a prism spectroscope, his wave-lengths 

 are arbitrary. A few years later, MascartJ called attention 

 to the series of doublets in the spectrum of sodium and to one 

 of triplets in the spectrum of magnesium. He thought there 

 was some connexion between the relative distances of the lines 

 in these similar groups, which could not be attributed to 

 chance, and which might point to some " harmonic'''' relations. 



In the same year M. Lecoq de Boisbaudran § announced, as 

 the result of his study of the flame-spectra of the alkalies and 

 the alkaline earths, a simple law. He selected certain lines 

 in each spectrum, which he called " homologous," as they 

 appear (with certain changes) in the spectra of homologous 

 substances, such as barium, calcium, and strontium. These 

 lines do not, however, have the same wave-length in the 

 different spectra; and his law was that the heavier the mole- 



* Communicated by the Author. 



t Pogg. Ann. cxxi. (1864) ; Phil. Mag. September 1864. 



X Comptes Rendus, lxix. (1869). 



§ Tom. cit. 



Phil. Mag. S. 5. Vol. 30. No. 182. July 1890. D 



