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The method described by Essen and Froome has been used to measure the refractive indices of water vapour, air, oxygen, nitrogen, hydrogen, deuterium and helium at a frequency of 9200 Mc/s. The value of (n-1)106 obtained for water vapour at 10 mm Hg pressure and 20°c is 60.7 which is the same as the value obtained at 24000 Mc/s, showing that the absorption line at 22000 Mc/s has a negligible effect on refractive index.
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L. Essen (1953) studied this question.
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