THEORY or POLYCONIC PROJECTIONS. 



11 



Since 

 But 



at the limit 



LOS'N= ZOSN+ ZS'NS. 



/LOS'M'- Z.OSN=~-dcp. 



S'M' = S'N=p-\-dp, 



^S^NS=^,= -^''^^ 



S'N p + dp 



Fig. 2.— Diflerential elements of a polyconic projectioiu 



