the Pressure of Radiation. KU 



In this case the mechanical bodily force is along Ox, and 

 is given by 



" "'■ v^y (22)* 



where 



Thus 



c 



r' = true current 

 ^ _ 1 BY 



~ ^TT dt ' 



47r^B^ 47rc'^ 'dt 



And i£ all the quantities vary as e''^'^\ 



B7__A B^Y^Y 

 47r ' B.^• 47r«VB.^B^ 'dt 



^ Arr-"^ ->^,n A rrr- .,2 ^2 



Now if all the interfaces are perpendicular to 0^', 7 and Y 

 are continuous throughout, and we see that the total mean 

 force per unit area upon any plate is equal to the difference 

 at the faces of the mean value of 



yl + ^i,(^Y (24) 



Consider an interface between transparent media ; then if 

 through the transition layer we have 



1 da; = 1 ; j 6 dx =p ; 



the boundary conditions become 



I By2 



and the differences are only of the order of the thickness of 

 the transition layer. Thus the only case in which there is a 

 mean bodily force along 0^ which sums up to a finite quantity 

 is when there is absorption of energy and consequent attenua- 

 tion of the waves. Consider an absorbing medium bounded 

 on the left by free aether and extending on the right to 



* Larmor, Joe. cit. 

 Phil. Mag. S. 6. Vol 6. No. 31. July 1903. M 



