76 THEORY OF COLLINEATIONS. 
tive projection appearing as a special case of such a non- 
perspective projection. 
93. Two Corresponding Conics, K and K'. Take as be- 
fore two planes x and 7’, intersecting in a linel. In the 
plane a draw any conic K touching / at L; and in 2’ another 
conic K’ also touching / but at another point, LZ’. From P, 
Fig. 11, any point in x not on K, draw two tangents to K; 
these will intersect / in two points, Q@ and R. From Q and R 
Fig. 11. 
draw tangents to K’ in a’; these will intersect in a point P’. 
Pand P’ in x and 7’ respectively are called a pair of corre- 
sponding points of the two planes. It is evident that this 
construction determines a one-to-one correspondence between 
the points not on K and K’ respectively of the two planes x 
and x’. 
Let P, be any point on the line PQ; it is evident from the 
construction that its corresponding point P,’ will lie on P’Q. 
