Accurate measurement of refraction and dispersion of a solid by a double-layer interferometer

A silica plate of plane-parallel faces is inserted into one gap of a double-layer interferometer that transmits white light to a prism spectrograph in order to produce elliptic rings of equal chromatic order (RECO's). The silica plate is rotated and the expanding RECO's are counted at their center while this center is coincident with a standard wavelength. An analytic formula that relates the fringe count to the rotated angle enables the refractive index of the rotated plate to be accurately determined for different wavelengths. The results are fitted to a single-term Sellmeier dispersion function to find the peak wavelength of the ultraviolet absorption band and the atomic number density for such a transition. The variation of either the dispersion coefficient or the group-velocity factor with wavelength is determined from either the displacement of the RECO center across the visible spectrum if one of the double-layer interferometer's mirrors is displaced parallel to itself or from measurements on the RECO diameters.