The justification for the rather full account this chapter proposes to give of Adam Hilger interferometers is that their introduction at the Works of Hilger Ltd established an epoch in the development of the firm. These interferometers received recognition in three unusual ways. The lens interferometer will provide quick and certain means of determining what departures from sphericity are required, and in what surfaces they are best applied, in order to reduce to a minimum, at any selected angle, the outstanding oblique spherical aberrations. The interferometers were used in Jena to an ever increasing degree for the testing of optical parts. The large interferometer for photographic objectives was used in the correction of the prisms of the tower spectograph of the Einstein Tower in Potsdam. The Mach-Zehnder interferometer may be mentioned for the sake of historical completeness. The Hilger Interferometers here described produce a series of interference rings which may be regarded as a "contour map" of the imperfections.