The established methods of interferometrically testing lens surfaces by comparison with a master surface are summarized, and the difficulties which arise in the case of steeply curved surfaces are explained. An instrument is described which enables highly curved spherical surfaces of numerical aperture up to 0.95 to be tested interferometrically at normal incidence. The method, which is non-contacting, allows the fringe pattern to be adjusted to the preferred form of straight line fringes, and obviates the risk of damage through scratching. Further, each master surface may be used with a variety of test surfaces of different radii of curvature. It is possible also to use the instrument in a subsidiary role as a relative spherometer accurate to about one micron with respect to the master surface, and as a Fizeau interferometer for the testing of less steeply curved surfaces and optical flats.