The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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Lens
Lens (Lat. lens, a small bean or lentil). An optical term given to discs of glass bounded generally by two spherical surfaces, or by a plane and a spherical surface. Sometimes, however, the surfaces are cylindrical, paraboloid, or hyperboloid. Lenses of a kind have been known from very remote antiquity, and Aristo-phanes (about 430 B.C.) writes of the transparent crystal
then used as a burning-glass to light fires. Whether this was of glass or of natural crystal is a matter of conjecture. Spectacle lenses are mentioned by Giordano da Rivalto in 1305, as having been invented only " twenty years ago." This will fix the date at 1285, when they were constructed by Salvino d'Armati, a Florentine. Alessandro della Spina, having seen some of Armati's spectacles, made some for himself, and published the method of manufacture. The manufacture was long almost con-fined to Italy, Sir Christopher Wren being one of the first to make satisfactory lenses in this country. In past times all lenses were made of crown or flint glass, the former being free from, and the latter containing, lead, the flint being slightly more refractive than the crown ; but since the introduction of the Jena glasses (see Glass) many of the optical glasses in use can neither be called flint nor crown. The sectional forms of the chief lenses are here given: -
Fig. 71. - a, double-convex ; b, plano-convex ; c, concavo-convex, or converging meniscus; d, double-concave ; e, plano-concave ;divergent meniscus.
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