A triple interferometer arrangement for determining the relative motion of opposing areas on the front and back surfaces of any vibrating plate is described and illustrated. The manner in which this interferometer is used to distinguish between flexural and longitudinal modes in quartz plates is also discussed and illustrated. These illustrations are selected to demonstrate several very interesting characteristics of an oscillating quartz plate. In the case of a quartz cube operated as an X-cut the influence of lateral vibrations is seen to affect seriously the computed frequency of the fundamental longitudinal waves propagated along X. A simple interferometer employed in the study of quartz plates is described, together with a strip method of excitation used in conjunction with this interferometer.
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