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Abstract

A gas refractometer is disclosed for use with a measurement interferometer and which in its simplest form (FIG. 1 ) comprises an auxiliary interferometer which uses the same light source as the measurement interferometer and which provides two light paths therethrough of different lengths. The gas refractometer operates as a tracking interferometer by providing a signal indicative of changes in the refractive index of the atmosphere in which the measurement interferometer is operating. At the same time the gas refractometer is made to act as an absolute refractometer by arranging that the maximum change in the difference between the two path lengths within the refractometer which will be produced for the full range of refractive index variation under which measurements are likely to be made will be less than one wavelength of the light used. Various embodiments of the refractometer are described including variants in which two auxiliary interferometers are used, and the use of the refractometer output to correct the measurement interferometer reading or to control the wavelength of the light used is also described.

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First Claim