It is pointed out that crystals of β-magnesia of optical quality may be obtained in sizes up to 1½ inches diameter. Windows for use in spectroscopic furnaces have been prepared from this material. Tests made with these windows show that they resist attack by the vapors of Ca, Na, K and Li. They transmit from 2150A in the ultraviolet to about 5 µ in the infrared. The index of refraction varies from 1.8450 at 2536A to 1.7313 at 7065A. The dispersion, ν, is 53.5. A preliminary investigation shows that it may be suitable for a quartz-magnesia doublet lens.
JOHN STRONG and R. T. BRICE, "Optical Properties of Magnesium Oxide," J. Opt. Soc. Am. 25, 207-207 (1935)