124 Mr GREEN, ON THE PROPAGATION OF LIGHT 



dx' dx dy dy dz dz' 



(dx') dx dz dx dz dx dz 



d#' </#' e?y' </y' dz dz' 

 (dx') dx dy dx dy dx dy 



V {U) + (is) + {dx)\\{dy-) + S|) + Kdy) 



or we may write 



' _ / _ ftff dx dy dy dz dz 

 dy dz dy dz dy dz ' 



., _ dx dx dy dy' dz dz' 



/3 = «c/3= -7--7-+-7 L -r- + j T-, 



ax dz dx dz dx dz 



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

 y = aby = -r- -T- + -f- -f- + -j— -v—. 

 ax dy dx dy dx dy 



Suppose now, as in a former paper, that (f> dx dy dz is the function 

 due to the mutual actions of the particles which compose the element 

 whose primitive volume = dx dy dz. Since must remain the same, 

 when the sides (dx 1 ) (dy) (dz) and the cosines a, /3, y of the angles of 

 the elementary oblique-angled parallelopiped remain unchanged, its most 

 general form must be 



<p = Function (a, b, c, a, (Z, y) 



or since a b and c are necessarily positive, also 



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



we may write 



<p=f(a\b\c* «',/3', 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 formula?. 



