( 591 ) 



Physics. — ''21ie abaorpliuti and enm^ion Unes of gaseous bodies." 

 Ky Prof. H. A. Lorkntz. 



(Communicated in the Meetings of November and December 1905). 



§ 1. The dispersion and absorption of light, as well as the 

 intluence of certain circumstances on the bands or lines of absorp- 

 tion, can be explained by means of the hypothesis that the molecules 

 of ponderable bodies contain small particles that are set in vibration 

 by the periodic forces existing in a beam of light or radiant heat. 

 The connexion between the two first mentioned phenomena forms the 

 subject of the theory of anomalous dis|)ersion that has been developed 

 by Sellmeyer, Boussinesq and Helmholtz, a theory that may readily 

 be reproduced in the language of electromagnetic theory, if the 

 small vibrating particles are supposed to have electric charges, so 

 that they may be called electrons. Among the changes in the lines 

 of absorption, those that are i)roduced by an exterior magnetic field 

 are of paramount interest. Voigt ') has proposed a theory which 

 not only accounts for these modifications, the inverse Zeeman effect 

 as it may properly be called, but from which he has been able to 

 deduce the existence of several other phenomena, which are closely 

 allied to the magnetic splitting of spectral lines, and which have 

 been investigated by Hallo ') and Geest ') in the Amsterdam labo- 

 ratory. In this theory of Voigt there is hardly any question of the 

 mechanism by which the phenomena are produced. I have shown 

 however that equations corresponding to his and from which the 

 same conclusions may be drawn, may be established on the basis 

 of the theory of electrons, if we confine ourselves to the simpler 

 cases. In what follows I shall give some further development to 

 my former considerations on the subject, somewhat simplifying them 

 at the same time by the introduction of the notation I have used 

 in my articles in the JMathematical Encyclopedia. 



1) W. VoiGT, Theorie der magneto-optischen Erscheinungen. Ann. Phys. Chem. 

 67 (1899), p. 345; Weiteres zur Tiieorie des ZEEMAN-ellectes, ibidem 68 (1899), 

 p. 352; Weiteres zm- Theorie der magnelo-oplischen Wirkungen. Ann. Phys., 1 

 (1900), p. 389. 



2) J. J. Hallo, La rotation magnétique du plan de polarisation dans le voisinage 

 d'une bande d'absorption, Arch. Néerl., (2), 10 (1905), p. 148. 



") J. Geest, La double refraction magnétique de Ia vapeur de sodium, Arch. 

 Néerl., (2), 10 (1905), p. 291. 



+) LoRExTz, Sur la theorie des pii.'nomènes magnéto-optiques rJcemment décou- 

 verts Rajiports prés. au Congres de physique, 1900, T. 3, p. 1. 



41 

 Proceedings Royal Acad. Amsterdam. Vol. VIII. 



