This paper describes a new instrument called a strobolyzer which makes it possible to observe, in slow motion, the vibrations of any body at any audio‐frequency even though the amplitude of motion is microscopic. It can be used to determine resonant modes of complicated bodies or to observe changes in motion as changes are made in frequency, loading, or driving point. The motion to be studied is detected by a number of vibration pickups placed to show the interesting features of the motion. The signal from each pickup is at the vibrating frequency but, in general, will differ from the other pickup signals in amplitude and phase. In the strobolyzer, the signal from each pickup is mixed with a common signal which differs by a small amount, Δcps, from the vibrating frequency. The signals from the mixer, after filtering, are at Δcps and the amplitude and phase relations which exist among the signals from the pickups are preserved in the signals from the mixers. The signal from each mixer drives an indicator which is mounted on a rough model of the vibrating body in a position which corresponds to the respective pickup.