350 KEPOBT — 1893. 



Recent Electrical Theories. 



12. A recent very comprehensive memoir ' by Drude, on this subject, 

 ^begins by alluding to the enormous rotatory power of magnetised bodies dis- 

 covered by Kundt, which places in strong light the direct magnetic origin 

 of the phenomenon of rotation, and to the observation of Kundt that a film 

 of non-magnetic metal deposited on a magnet destroys Kerr's phenomena, 

 so that they cannot be due to magnetic rotation in the air. He also re- 

 marks on the insufficiency of the notion that before reflexion the light 

 penetrates slightly into the magnet and so undergoes rotation in its sub- 

 stance ; this notion is in the first place not precise or quantitative at all, 

 and further it assigns to the surface layer of transition an influence in 

 reflexion which is much too great in view of other optical phenomena. 



He then takes up one type of the formal equations of propagation ia 

 an isotropic medium, viz. (v, v, iv) being a certain vector (the rotation in 

 an isotropic elastic medium) 



— (u, V, iv)=EV^(u, V, w) ; 



and he works out as follows the results of adding on to the right-hand 

 side terms of the various kinds originally suggested by Airy. On adding 

 terms of the form which represents the theory of C. Neumann, viz. 



"there come equations of the type 



d^u „, , ■, dv ■, dw , d\ 



to which the term ^- is conjoined in order to allow us to have 



dx 



du dv dw ^ 



dx dy dz ' 



i.e., in order that the light- waves shall remain purely transversal. 



The form of this new term might be derived from the general varia- 

 tional equation of motion of the system, worked out subject to this limit- 

 ation of the displacement (u, v, w). 

 On writing 



ry ^s fdw_dv du__dw dv _du\ 



\dy dz' dz dx' dx dy) 

 there follows 



''^=-(»-i+''§+»3f)- 



and also the equations of propagation in the form 

 dt"^ 



,V'»=.V^V'»-|(..| + 5,|+.,f). 



* p. Drude, • Ueber magneto-optische Erscheinungen,' Wied. Ann., xM. 1892. 



