162 Mr Glazebroulc, On some equations connected with [Nov. 28, 



471-$' 



- ,- in direction of displacement, 



and Tr , tan % along the wave normal. 

 The electrostatic energy per unit of volume is 



*{ 



fP + gQ + hR) 



= 27r L£+£ + Z 



= 2tt/x. {aV + &y + cV] S 2 



= 2ir^V 2 S 2 (17). 



The electro-kinetic energy is 



£-(a 2 + /3 2 + 7 2 ) 



= 27r/xF 2 >Sf 2 (18), 



And as before with isotropic media the two are equal. 



We proceed now to consider the reflexion and refraction of a 

 plane wave incident on a crystalline surface. 



Take a unit sphere and let a plane parallel to the face of 



incidence cut it in BCB'. The incident reflected and refracted 

 waves cut the face of incidence in the same line. 





