12 



Ellery W. Davis 



P s belonging to the line P 1 P 2 whose blue point is at the crossing 

 of blue line and black line has its black point on the black line 

 at the distance / equal to the projection parallel to P t P. 2 of P r 

 and P 2 upon the black line. Moreover, mis point bisects the 



Fig. 12. 



line drawn from the center P we seek and terminated by the 

 blue line. 



Fig. 13 illustrates how given the center F and a red vector 

 P 1 we can construct other vectors P 2 and P z .. 



Fig. 13. 



To see more clearly and comprehensively the character of the 

 central line consider the special line 



3' = ix, 



whose center is the origin. 



At once 



y' = — x" and y"=x'. 



Hence the red vector is at risrht. angles to the black vector from 



12 



