Abstract : The refractive index of a sample of hot-forged CaF2 was measured from 0.25 micrometers to 8.0 micrometers by means of the minimum-deviation method on a precision spectrometer. Data were obtained near 20 C and 34 C. Each data set was fitted to a three-term Sellmeier-type dispersion equation, which permits interpolation of refractive index as a function of wavelength within a few parts in 100000. With the index values obtained at the two temperatures, the change, in index with temperature was calculated. The refractive index and values obtained for this specimen are compared with data previously published. Two new experimental arrangements have been constructed for the interferometric measurement of the thermal coefficient of refractive index, thermal expansion, and piezo-optic constants. The first permits measuring dn/dT and thermal expansion from -180 C to +200 C. The second is a highly stable and sensitive interferometer for measuring photoelastic constants in the visible, the near infrared, and the near ultraviolet. Thermal expansion data are obtained on CVD ZnSe, CaF2 and BaF2 between -100 C and +120 C and fitted to a third degree polynominal in temperature. The dn/dT for CVD ZnSe at 632.8 nm over the same temperature range was obtained. The piezo-optic constants of fused silica and CVD ZnSe obtained with the new interferometer at 632.8 nm are in excellent agreement with values obtained by other methods. (Author)