Show that its polar lines with respect to two given Conics generate a point-row of the second order.
"An Elementary Course in Synthetic Projective Geometry"
Lehmer, Derrick Norman
We have, then, the beautiful theorem due to Desargues: The system of Conics through four points meets any line in the plane in pairs of points in involution.
"An Elementary Course in Synthetic Projective Geometry"
Lehmer, Derrick Norman
We may therefore infer the theorem Through four fixed points in the plane two Conics or none may be drawn tangent to any given line.
"An Elementary Course in Synthetic Projective Geometry"
Lehmer, Derrick Norman