138.  Involution of rays determined by a conic. We have seen in the theory of poles and polars (§ 103) that if a point P moves along a line m, then the polar of P revolves about a point.  This pencil cuts out on m another point-row P’, projective also to P.  Since the polar of P passes through P’, the polar of P’ also passes through P, so that the correspondence between P and P’ is double.  The two point-rows are therefore in involution, and the double points, if any exist, are the points where the line m meets the conic.  A similar involution of rays may be found at any point in the plane, corresponding rays passing each through the pole of the other.  We have called such points and rays conjugate with respect to the conic (§ 100).  We may then state the following important theorem: 

139. A conic determines on every line in its plane an involution of points, corresponding points in the involution _ being conjugate with respect to the conic.  The double points, if any exist, are the points where the line meets the conic._

140. The dual theorem reads:  A conic determines at every point in the plane an involution of rays, corresponding rays being conjugate with respect to the conic.  The double rays, if any exist, are the tangents from the point to the conic.

PROBLEMS

1.  Two lines are drawn through a point on a conic so as always to make right angles with each other.  Show that the lines joining the points where they meet the conic again all pass through a fixed point.

2.  Two lines are drawn through a fixed point on a conic so as always to make equal angles with the tangent at that point.  Show that the lines joining the two points where the lines meet the conic again all pass through a fixed point.

3.  Four lines divide the plane into a certain number of regions.  Determine for each region whether two conics or none may be drawn to pass through points of it and also to be tangent to the four lines.

4.  If a variable quadrangle move in such a way as always to remain inscribed in a fixed conic, while three of its sides turn each around one of three fixed collinear points, then the fourth will also turn around a fourth fixed point collinear with the other three.

5.  State and prove the dual of problem 4.

6.  Extend problem 4 as follows:  If a variable polygon of an even number of sides move in such a way as always to remain inscribed in a fixed conic, while all its sides but one pass through as many fixed collinear points, then the last side will also pass through a fixed point collinear with the others.

7.  If a triangle QRS be inscribed in a conic, and if a transversal s meet two of its sides in A and A’, the third side and the tangent at the opposite vertex in B and B’, and the conic itself in C and C’, then AA’, BB’, CC’ are three pairs of points in an involution.