A curious and interesting gem is Frégier's theorem, quoted here from David Wells:

Choose any point $P$ on a conic, and make it the vertex of a right angle which rotates about $P$. Then the lines through the points of intersection, $AA$, $BB$, and so on, will all pass through a fixed point $X$ which lies on the normal at $P$, that is, on the line through $P$ perpendicular to the tangent at $P$.

Who was Frégier, and where can I find the earliest publication of his theorem?