From an original suggestion of Tolansky (1943) an interferometer system is described capable of high absolute accuracy in the measurement of small linear displacements. Because divergent light beams are used a great simplification may be made in the optical train and the use of a laser light source permits automatic fringe detection and recording against high background light intensities. An apparatus is described for the absolute measurement of the thermal expansion of copper to 1200K and it is pointed out that the apparatus is capable of measurements to still higher temperatures.