296 ON THE PROPAGATION OF LIGHT 



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dx dz dx dz dx dz 



Tx 



dx dx dy dy dz dz 



dx\ dx dy dx dy dx dy 



y <y >y 



or we may write 



, , dx' dx dy dy dz dz' 

 a=&ca = - r -- r -+-f- ir- + j--j-> 

 dy dz dy dz dy dz 



dx' dx' ,dy' dy' dz'dz' 

 p = acp = -j- -jr- + -f- -j- + -j- -j- , 

 ax dz ax dz dx dz 



, , dx' dx dy' dy dz' dz' 

 ry' = aby = - T -- r + -f- -+-J- -7- . 

 dx dy dx dy dx dy 



Suppose now, as in a former paper, that fydxdydz is the 

 function due to the mutual actions of the particles which com- 

 pose the element whose primitive volume = dxdydz. Since </> 

 must remain the same, when the sides (dx}, (dy'}, (dz'} and the 

 cosines a, /3, 7 of the angles of the elementary oblique-angled 

 parallelepiped remain unchanged, its most general form must be 



< = function (a, b, c, a, /3, 7), 



or since a, b, and c are necessarily positive, also 



a' = ca, /3' = ac/3, and y=aby, 

 we may write </>=/(a 2 , & 2 , c 2 , a', ff, 7') .................. (1). 



This expression is the equivalent of the one immediately 

 preceding, and is here adopted for the sake of introducing 

 greater symmetry into our formulae. 



We will in the first place suppose that <f> is symmetrical 

 with regard to three planes at right angles to each other, which 

 we shall take as the co-ordinate planes. The condition of sym- 



