AND REFRACTION OF LIGHT. 



or — «, — w, —a, — /3 

 or - y, - v, -a, - 7 

 or — x, — u, — (3, — 7 



8, w, a, /3 

 are written for \ y, v, a, y 

 x, u, /3, 7, 



In this way the 21 coefficients are reduced to 9, and the resulting 

 function is of the form 



*& 



+ H (% 



l(~y + La !! + M^ + Ny 2 



or ,dv dw Q du dw „ du dv 



dy ' dx dx' dz dx ' dy ^ 2 ' 



(A). 



Probably the function just obtained may belong to those crystals 

 which have three axes of elasticity at right angles to each other. 



Suppose now we further restrict the generality of our function by 

 making it symmetrical all round one axis, as that of 8 for instance. 

 By shifting the axis of x through the infinitely small angle $6 ; 



and 



Making these substitutions in (A), we see that the form of 2 will 

 not remain the same for the new axes, unless 



L = M, 

 P=Q; 



