164 Mr. T. H. Havelock on 



substance constituted of systems of point charges. If in the 

 latter case instead of (22) we had taken the mechanical force 

 to be given by v<ylc, where v is the total current, we should 

 have obtained a different result ; in fact, working this out, we 

 find instead of (33) 



which is quite analogous to (16). 



§ 9. We shall consider now the general case of oblique 

 incidence, the plane of separation being the plane of yz. 

 Then in the absorbing medium to the right of this plane there 

 is a bodily force along Ox given by 



where 



(:w) 



(./V.''')=^(X,Y,Z)I 



There is also a surface traction toward the absorbing medium 

 given by — 27r?i^, where n is the normal component of the 

 material polarization (/', g'^ //)*. 



Using the general circuital relations (18) and (19), we find 

 that for transverse waves in the medium F reduces to 



^ = -8^^..(^+^+^)-8^^^B^.Ub^-) -^Kw) ^(b^) j'^^^^ 



Then, as before, integrating F along Ox from a point just 

 inside the surface to a point at which the disturbance vanishes, 

 and adding the surface traction, we see that the mechanical 

 action upon the medium along Ox may be expressed as a 

 pressure on the surface given by 



p=A r^2^_^2 + ^2 + x2 + Y24Z^-(e-l)2XT . (38) 



where the mean values of the terms are taken just inside the 

 surface. Now a, /3, 7, eX, Y, and Z are continuous at the 

 surface ; hence 



^^=8^ 



2-. 



a'^ + yS^ + 7^ + Y2 + Z2+^X2l . . . (39) 



where the values of the forces are those just outside the 

 surface. 



* Larmor, loc. cit. 



