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A method employing both multiple-beam and two-beam interferometry for the measurement of extremely small changes in thickness and refractive index of glass plates is described. The glass plate which is to be measured is scanned through a narrow laser beam in one arm of a Twyman-Green interferometer, the output of which constitutes one arm of a two-beam optical measuring set. By measuring the changes in intensity of the light emerging from the interferometer (once with the interferometer mirrors in use and once with the mirrors blocked), two sets of data are obtained which permit the separate determination of the variations of thickness and refractive index. Since the optical measuring set can easily detect changes in light intensity of one part in 10-3, changes in thickness of the order of and changes in refractive index of the order of 10-7can be measured. By scanning over the area of the sample, a contour diagram depicting both thickness variations and refractive index variations can be constructed.