Methods and apparatus that combine dispersion interferometry with refractometry to compensate for refractive index fluctuations in the measurement path of a dispersion interferometer over both short and long time periods. Dispersion and refractometry data are weighted over appropriate time intervals, and means and methods are also provided for initializing &Ggr;, the inverse dispersive power, so that the dispersion and refractometry data are self consistent. A refractometer is placed in close proximity to the measurement path of the dispersion interferometer to experience substantially the same air flow and act as a surrogate for obtaining information about the index of refraction.