French physicist who invented the reflecting telescope that bears his name. Cassegrain reflectors employ a concave primary mirror to focus and reflect light onto a convex secondary mirror. The light is then reflected back through a hole in the primary mirror to a lens. Cassegrain's design increased angular magnification over other types of reflectors. A more advantageous feature, not recognized for another century, is the elimination of spherical aberration inherent in other two-mirror reflectors.