A strain meter has been designed embodying techniques not used in earlier models. Minimum use of heavy and permanent auxiliary fixtures has made possible a portable instrument in which no components need be abandoned in order to change location. Provision for frequent or continuous interferometric calibration, and a photographic record therof, has provided uninterrupted strain measurements with long-term dependability. Also included is the usual continuous low-speed ink recorder for tidal and secular variations, and a high-gain band-pass recorder for the observation of long period waves and free oscillations of the earth. Details are given describing the method used for interpreting the interferometer image and its conversion to strain measurement.