THEORY OF POLYCOXIC PROJECTIOXS. 



155 



two surfaces are thus divided into infinitesimal rectangles 

 which correspond the one to the other. 



Fig. 42.— Two tangents at right angles and their projections. 



This fact being established, let ilf be a point in figure 43 

 infinitely near to upon the first surface and let OPMQ 

 be that one of the infinitesimal rectangles which we have 

 just described that has OM as a diagonal. Let us move 



Q' 



M* 



O ' P P' 



Fig. 43.— Projection of infinitely near points. 



the second surface and place it so that the projections of 

 the sides OP and OQ faU upon the sides themselves pro- 



