It is pointed out that crystals of β-magnesia of optical quality may be obtained in sizes up to 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.
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