A technique of measuring the refractive index of a plane parallel test plate has been investigated and further developed. The refractive index of the test plate, referred to that of the surrounding medium, can be obtained by measuring the optical-path differences introduced between a ray through and a reference beam outside the plate by two successive rotations, which change the angle of incidence. Close parallelism between the polished surfaces of the test plate is not required but one of the surfaces must be of good optical quality. The index of refraction, relative to that of the surrounding medium, may be calculated without knowledge of the thickness of the plate or of the wavelength of the light Two series of measurements show that the accuracy of the refractive index of a fused-quartz plate, relative to that of surrounding air, is better than 4×10−6. If the same plate is immersed in water, the accuracy turns out to be about 1×10−6. These results were obtained with a commercially available goniometer, used as the angle-recording instrument. This is the part of the equipment that largely determines the accuracy of the index of refraction.
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