N. Y. AcAo^MV 

 OF Scife N 



PROCEEDINGS 



OF 



THE ROYAL IRISH ACADEMY 



PAPERS READ BEFORE THE ACADEMY. 



I. 



THE IONIC THEORIES OF MAGNETO-OPTIC ROTATION. 



Br FELIX E. HACKETT, M.A., B.Sc, 

 University College, Dublin. 



Read Apkil 23. Ordered for Publication April 25. Published July 7, 1906. 



The influence of a magnetic field on the transmission of plane- 

 polarised light has always been an important problem in any theory 

 that endeavoured to explain the interaction of light and matter. The 

 earlier attempts to explain the phenomenon are all deficient in that 

 they have no real physical basis ; but, with the rise of the theory of 

 electrons, great progi'ess has been made, especially by Drude, who 

 lias developed two possible explanations of the phenomenon on what 

 may be called the ionic hypothesis. These two explanations still hold 

 the field, as the existing experimental evidence, and in particular 

 the evidence obtained to test the formulae for the rotation of the plane 

 of polarisation, has not enabled either to be decisively rejected. The 

 principal criterion to decide between them is this — that the hypothesis 

 which assigns the effect to rotating ions indicates that the magnetic 

 rotation should change sign when crossing an absorption-band, while 

 the second formula, based on the Hall-effect, leads to the conclusion 

 that the rotation should be of the same sign on each side of an 

 absoi-ption-band. The latter condition is satisfied completely in the 

 case of sodium vapour, which has been very completely investigated 

 by AVood.^ Its behaviour has also been found to satisfy some other 



1 Phil. Mag., Oct., 1905. 



U.I. A. I'UOC, VOL. XXVI., SECT. A.] 



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