An air refractometer has been developed for accurate measurements of the refractive index of gases under well-defined conditions. Measurements have been performed for dry and moist air at four wavelengths distributed over the visible part of the spectrum. The measured values have been compared with values determined by modified Edlén's formulae and measured parameters. A fitting procedure is described to adjust the humidity dependence and the constant term of the dispersion formula so that optimum agreement is obtained. A standard deviation of 3 10 -9 for the residuals describes the reproducibility of the measurements and the success of fitting. For 50 % relative humidity, a standard uncertainty of 10 -8 for the fitted formulae is claimed, arising mainly from the uncertainty contributions of the air parameters.

For accurate interferometric length measurements, the accurately determined value of the refractive index of air is required. In many cases the refractive index is evaluated from measured air parameters. A widely applied set of formulae for this evaluation has been provided by Edlén who fitted existing experimental data. Modified Edlën's formulae are presented which were fitted to values obtained by an air refractometer situated in an environmental chamber. This chamber allows stabilization of the air parameters. A new series of measurements has been performed with three laser wavelengths at 543 nm, 633 nm, and 780 nm and good agreement with the results previously reported has been obtained.

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