( 602 ) 



to the supposition that for the paths of the electrons there exist 

 conditions (fields) of stability, which are deterniijied by the temj)e- 

 ratiire. The action of the magnetic force and the change in the rate of 

 vibration would then bring about that some electrons enter these 

 fields of stability or leave them, both changes occurring either in the 

 direction of greater union with or further separation from the centres 

 which determine the paths, and the increase of this action at low 

 temperature would be in connection with the small velocity. The 

 influence on the stability of the paths, which is here considered, 

 would be the same as manifests itself in the change by temperature 

 of the number of electrons (see § 7) which satisfy the cojiditions of 

 the motions which may be ascribed to (juasi-elastic forces. 



In this connection the (juestion suggests itself if the greater sta- 

 bility of vibrations in a certain direction will not give rise to 

 paramagnetic properties. 



§ 14. VariahUity of the mass of the electrons ivith the direction 

 of the movement. The theory of the magneto- optic phenomena in 

 crystals (Voigt ^), Jean Becquerkl '■')) leads to the following results. 



The magnetic field gives rise to certain connections between the 

 motions of the electrons in the different princij)al directions of the crystal. 

 Let us consider the simple case which is repeatedly met with, viz. that 

 the corresponding bands in the different spectra occupy the same place. 

 In that case according to the theory the magnetic doublets will have to be 

 symmetrical, and when the hands are sufficiently narrow to allow 

 us to neglect the breadth, the deviations will be proj)orti()nal to the 

 square root of the product of the two magnetic constants which 

 belong to the corresponding bands of the two spectra. If the beam 

 of light and one of the i)riucipal directions 1, 2, 3 of the crystal 

 are made to coincide with the direction of the magnetic field, those 

 two of the three spectra of the crystal are observed which correspond 

 with the vibrations normal to the lines of foi'ce. 



Obser\ation shows that both for the uniaxial crystals of xenotime 

 and tysonite and for the biaxial crystals of didymium sulphate, 

 neodymium sulphate, and praseodymium sulphate (which last exhibits 

 some lines in liquid hydrogen as sharp as vapour lines) the doublets 

 of the common band have the same divergence. A phenomenon of 

 great importance is observed, when the spectra of vibrations normal 

 to the lines of force are combined in different Avays. If the 

 directions 1, 2, 3 successively are placed in the direction of the 



^) Nachr. Kön. Ges. d. Wiss. Göttingen Juli 1906. 



^') G.R. 19 Nov. 3. 10. 24 Dec. J 906. Radium IV n'. 3 Mars 1907. 



