After defining optical glass, the requirements of the optical industry are discussed from the standpoints of quantity, quality, and variety. Consumption is small but the demands in respect of quality are high. The variety of optical constants now available is great but the designer is in need of combinations of index of refraction and dispersion not now available.
W. B. RAYTON, "Optical Glass Requirements of the Optical Industry," J. Opt. Soc. Am. 28, 8-8 (1938)