A recorder used for making ink records of low frequency oscillations of a small rotating mirror (such as, for example, a galvanometer mirror) is described: a light beam, modulated in accordance with the mirror vibrations, excites a photoelectric cell, the output of which is sufficiently amplified by a low frequency amplifier to control a pen writer. Since the instrument has been developed for use in conjunction with photographically recording seismographs, the treatment in the article is from a seismological point of view. The frequency response of the recorder alone is calculated, and the method of obtaining it for the entire system of the recorder in combination with a given seismograph is indicated. Records are shown to demonstrate the fidelity of reproduction.