112 INORGANIC EVOLUTION. [CHAP. 



complex types, while others again remained almost if not altogether 

 unchanged. 



And then there was another revelation. 



Not only do lines in the spectra of different substances vary in this 

 respect, but lines in the spectrum of any one substance are differently 

 changed ; while some spectral lines of an element show a considerable 

 resolution in the magnetic field, others are scarcely affected at all. 

 This important fact was first stated by Dr. Preston in 1897.* 



This brings us to the connection between this line of work and my 

 own, for we now find lines of the same substance behaving differently 

 qua magnetic -perturbations, as I found iron lines behaving differently 

 in the spectra of sun spots qua velocity. About this different behaviour 

 qua perturbation there is no question. I will refer to some of the work 

 since done in this connection. 



M. Cornu was the next to note the importance of this. He 

 writes : 



" The effect of the magnetic field on the period of vibration of the 

 radiations of the luminous source seems to depend, not only upon the 

 chemical nature of the source, but also upon the nature of the group of 

 spectral lines to which each radiation belongs, and on the part which it plays 

 in this group."! Somewhat later MM. H. Becquerel and Deslandres 

 gave details with regard to .the spectrum of iron in the ultra-violet 

 region, calling attention to these observations as being of great import- 

 ance " physically, chemically, and astronomically. "J 



Dr. Zeeman subsequently published the statement that observing 

 across and along the lines of force, although the vast majority 

 of iron lines were, with the field used, resolved into doublets, 

 triplets, quadruplets, &c., three or four lines seemed unaffected. In the 

 case of a few lines he further found inequality between the outer com- 

 ponents of a triplet across, and of the corresponding doublet along, 

 the lines of force. 



Messrs. Ames, Earhart, and Reese next noticed further peculiarities 

 about the behaviour of some of the iron lines. || 



When the radiation at right angles to the magnetic field was studied, 

 each line in the spectrum was found in general to be broken up into 

 three, the central component being plane polarized with its vibrations 

 -along the line of force, the two side components being plane polarized 



* Trans. Hoy. Dub. Soc., vol. vi, p. 385 (1898), and vol. vii, p. 7 (1899). 

 f Astrophysical Journal, vol. vii, p. 163, 1898. 

 J Comptes Rendus, vol. cxxvi, p. 997 ; vol. cxxvii, p. 18. 



Proc. of Roy. Acad. of Sciences, Amsterdam, June 25, 1898, and Astro- 

 physical Journal, vol. ix, p. 47. 



|j Astrophysical Journal, vol. viii, p. 48. 



