The Dispersion of Air Between 2500A and 6500A

The available experimental data on the optical dispersion of air have recently been critically assessed by Meggers and Kessler. They find that none is satisfactorily reliable when subjected to a test depending upon the application of the Ritz combination principle to term analysis in the first spectrum of 19880Hg lying between 2536A and 5791A. In the present paper, a new dispersion curve for air is derived from reasonably concordant refractivity data previously obtained in the visible spectrum at three different laboratories. The cauchy equation of this curve is used to reduce to vacuum wave numbers the highly precise wavelengths in air of 27 lines of 19880Hg measured by Meggers and Kessler. If the truth of the combination principle is assumed, the constancy of recurring term intervals between 2536A and 5791A shows that the new dispersion equation for air is at least as reliable as the original wavelength measurements, for which an average accuracy of ±0.0001A was claimed. Alternative forms of the general refractivity—density relationship are quoted which render the new dispersion equation suitable for practical use in spectroscopy and length metrology.