Abstract: Apparatus for testing circular cylindrical or conical specularly reflective surfaces of pieces, e.g. optical elements or metal components, to be tested. A collimated light beam from a Fizeau interferometer is directed axially onto a specularly reflective conical surface of a member which reflects the light onto the surface to be tested. The surface to be tested, directly or indirectly, reflects the light onto the conical specularly reflective surface which, in turn, reflects it back towards the interferometer. Information on the trueness of the surface to be tested can be gained from inspection of the pattern of the interference between light projected by the interferometer and light reflected back to the interferometer.