Anomalous dispersion of birefringence of sapphire and magnesium fluoride in the vacuum ultraviolet

Previous measurements of the birefringence of sapphire and magnesium fluoride have been extended to their respective limits of transparency, revealing anomalous dispersion effects at short wavelengths, which are qualitatively discussed. Possible practical applications to Lyot filters and quarter-wave plates for Lyman-alpha are indicated.