Dynamical Theory of Diffraction. 225 



medium of permeability //, is 



±L(o? + /3*+<y*)dxdydz, .... (3) 



where a, /?, 7 are the components o£ magnetic force, while 

 the electrostatic energy is 



^{f+f+h?)dxdydz, .... (4) 



f,g, h being electric polarization and K the specific inductive 

 capacity. 



2. On the mode of identification suggested by Larmor, we 

 may, therefore, take 



0, ft 7) proportional to (f, 77, J) = ^ (f , 77, J) 



and (/, g, h) „ „ (<d x , w y , co z ), 



the constants being suitably adjusted so that 



*=/=3, &c ., 

 which implies 1 fa fir . 



v%rv«> w 



or the velocity ( = — =-) of electromagnetic disturbance is 



equal to a /-' = that of the rotational wave in the elastic 



medium. 



This also satisfies the usual relations in elasticity and 

 electromagnetism, since 



and 



*• @-g) 



_/dY_B£\ 



and further implies the condition 



£+■■ •+••■-«. 



since 



d# By a* 

 P/«7. M«7. S. 6. Vol. 28. No. 164. i%. 1914. Q 



