view Abstract Citations References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Crystals and crystal lenses. Henroteau, F. Abstract M any materials have been used for the making of lenses. Cood optical glass, especially in large pieces, is expensive to produce and in aspherical lenses, such as Schmidt correctors, is long and tedious to grind and to polish. Recently plastics have been used but they are subject to strains and are easily scratched and deformed. More- over, cast plastic lenses could never have the accuracy of glass ones for astronomical purposes, because having to be made at fairly high temperatures, they are somewhat distorted and have a slightly different refractive index after cooling. Transparent crystals, grown to any size, are absolutely homogenous and have constant refractive indices. Many crystals, moreover, are transparent in a much wider range of the spectrum than glass and can be used for many applications in the infrared and in the ultraviolet. Beautiful crystals of sodium chloride, potassium bromide, lithium fluoride and other chemicals crystallizing in the cubic system have been produced by special methods involving fusion followed by slow cooling. Methods are being developed by the writer and his associates in the laboratories of the Farnsworth Television and Radio Corporation, in Fort Wayne, Indiana, to produce crystals from solutions in water. A large, single crystal of ammonium alum, weighing more than two kilograms, was recently obtained and could have been develo~ed by accretion to wei~h much more than that. Also an apparatus has just been completed by means of which lenses can be made out of soluble cubic crystals, using controlled dissolution. The principle of this apparatus is the following: A negative form of the lens surface, spherical, aspherical or even skew is made out of insoluble material. This form is cooled and moist air is blown toward it with the result that a very thin film of moisture, perhaps one micron thick, is formed over it. A crystal, say of sodium chloride, attached to a rigid stem which moves back and forth by means of a motor is brought into contact with this film and then removed. A thickness of the order of one micron is dissolved where the crystal was in contact with the film. The dissolved part is then blown off by a strong blast of air which also removes the rest of the original film on the form. A new film is then produced on the form and the operation just described is repeated again and again until the outside surface of the crystal has taken the exact shape of the form. Farnsworth Television and Radio Corporation, Fort Wayne, Ind. Publication: The Astronomical Journal Pub Date: June 1945 DOI: 10.1086/105849 Bibcode: 1945AJ.....51..122H full text sources Publisher | ADS |