50 PROFESSOR KELLAND ON THE POLARIZATION OF LIGHT 



/e* MLflfTx a /c 2 _, M' dT\ 



= - I^T -- '.-}cos(p t cos6+ (^ T- ) 



\2 e, dx) \2 e' dx ) 



F 



^ 

 f dy 



Therefore by addition, 



<? M/rfl rfR\ FrfR 



cos < 



M/rfl rfR\ FrfR, 



( + ) COS0 + - -' 

 e \dx dx/ f dy 



-o - 



2 e \dx dx/ f dy 



rf 2 "V 



To find the value of -n. 



df 



By interchanging z and a;, ^ and x", 7 and a, y and a' in equation (5), we 

 obtain 



... (12). 



r 



NOW 2(0r + I 

 r 



' cos a') + -'sin0 + 



6 $ X (t X 



and all the other quantities in equation (12) are zero. 

 Hence 



M fdl' dR'\ M,rfT . /, M'rfT 



+ -(- -- -j ) + '-j sm^ + -c 

 e \dx dx ) e, dx d dx 



We proceed now to find the equations of motion of a particle situated within 

 the crystal. Retaining the preceding notation, the resolved part of the force pa- 

 rallel to the axis of x is 



+ 2 



