The refractive indices of CsI were determined at three temperatures near 15, 24, and 34°C for wavelengths from 0.29 micron in the ultraviolet to 53 microns in the infrared. The thermal coefficients of index were determined for each wavelength and all data were reduced to 24°C. A dispersion equation of the Sellmeier type having five terms was fitted to these data. Four of the terms contained absorption wavelengths which were either known experimentally or had been computed from the lattice constants of the crystal. The fifth was adjusted to fit the data and has a value of about 0.02 micron. The formula was extrapolated to infinite wavelength, and the value of n∞2 agreed with the known dielectric constant at 30 cm. The dispersion of CsI for infrared wavelengths from about 30 to 40 microns is less than that of CsBr but only by a factor of about 2. At 50 microns the dispersion of CsI is practically equal to the 40-micron dispersion of CsBr.
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