200 Mr Dixon, On a method, etc. [May 25, 1896. 



From these eliminate c. Thus 



a (nj, — njj) (w^ + n. 2 l 2 ) = b (m^ — m^i) (m^ + m,n,), 

 or cikfxv = bXfMv. 



By symmetry each = c\/j,i'. 



Thus, unless the surface is a sphere, X or jjl or v = 0. 



These examples will perhaps be enough to illustrate the 

 method, which is an extension of that of Joachimsthal, so freely 

 used by Dr Salmon. 



