The so-called group index of refraction may be measured by observing the shift of the visibility envelope when a sample is inserted into one arm of a Twyman–Green interferometer. Although the group and phase index curves are related, the phase index cannot be directly calculated from the group index because there are infinitely many phase index curves that correspond to any given group index curve. A method of selecting the correct dispersion curve from the family of possibilities is presented, using a priori information based on common properties of known glasses. This allows the phase index to be calculated from the group index to an accuracy of ~0.0016 across the visible spectrum.