A method of determining the shape of a concave mirror; also known as the knife-edge test. A pinhole light source is placed at the mirror’s radius of curvature, and a straight edge (e.g. a knife-edge) adjacent to the pinhole. The mirror is illuminated by light from the pinhole. If the mirror is spheroidal it will produce a sharp image of the pinhole at the knife-edge, and the tester will see the mirror’s illumination abruptly cut off as the knife-edge is moved across the image. Any small irregularities in the figure show up clearly as humps or hollows. A paraboloid appears like a doughnut, deeper at the centre than at the edges. The test was devised by the French physicist Jean Bernard Léon Foucault (1819–68).