A more sensitive method for observing longitudinal modes in vibrating piezoelectric plates consists in measuring the changes in the optical path presented by these plates to a beam of light. This is simply and conveniently accomplished by placing the transparent piezoelectric plate in one arm of a Michelson interferometer so that one of the interfering beams passes through the plate. Where a Michelson interferometer is not available, this may be accomplished in the case of a thin plate by means of the thick‐film fringes formed by reflecting a beam of light from the two major surfaces of the plate. Changes in the optical path are dominated by the changes in the index of refraction of the vibrating plate. The variations of the index of refraction are markedly greater for the extraordinary ray of a light beam propagated along theY‐axis of aY‐cut quartz plate in which the compressional wave trains are propagated along the X‐axis. Preliminary observations regarding the variation of the index of refraction in other crystal cuts are also presented. A brief summary is given of the longitudinal modes which have been observed in plates of quartz and tourmaline.