IN the experimental investigation of the problem of isolating very sensitive measuring instruments from the effects of building vibration, it has been found desirable to measure amplitudes of the order of 0.25 µ (10−5 in.). Optical interference methods have been investigated and used by Thomas and Warren1, Osterberg2, Cortez3, Kennedy4 and other workers. Multiple-beam interference methods, however, have not been applied by these workers. Tolansky and Bardsley5 have demonstrated the use of multiple-beam methods in studying vibration in quartz crystals. The use of multiple-beam interference techniques in conjunction with stroboscopic techniques offers a method of calibrating the most sensitive vibration detectors in terms of light waves to a high precision.