Abstract: An optical seismograph which functions to measure the relative movement of a beam of electromagnetic waves connected to the extremities of a plurality of pendulums and includes a collimated source which delivers a parallel beam of electromagnetic waves and a plurality of pendulums whose oscillation paths are perpendicular to one another, means fixed in relation to the electromagnetic wave beam direct the luminous beam on the heavy extremities of the various pendulums; and photoelectric means to measure the position variations of the extremities of the pendulums in relation to the electromagnetic wave beam. The seismograph is suitable for the detection of earthquakes.