HISTORIC CAMERA

History of the Photographic Lens





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DICTIONARY OF PHOTOGRAPHY
1889, London
by E.J. Wall



Lens. An optical term given to disks of glass bounded by two spherical surfaces, or by a plane and a spherical surface. A true lens is one which has the form shown in fig. 1, a; but the name now includes many other shaped glasses or combination of glasses from the analogy of their action upon light. The first mention of the use of a lens which I have been enabled to trace out is by the Chinese moralist Confucius, 748 B.C., who says, " As we use a glass to examine objects, so must we look to the present for futurity." But in the Western classics, about A.D. 40, mention is made by Seneca, Aristophanes, and other writers for the first time of globes of water and globes of glass; but of a true lens absolutely no mention is made, and it is even doubtful whether they were more than conversant with the burning powers of the above globes. There is, however, in the Assyrian Section of the British Museum a piece of rock-crystal of piano-convex form, which Sir David Brewster states was designed for magnifying. It has been shaped oval, evidently by a process of chipping and grinding, and both plane and convex surfaces have been partly polished. It seems more likely however, that it was used as an ornament. The date of this is about 720 B.C. There are also in the British Section several antique glass bosses, which have been evidently polished and cut to a wonderfully true curve. These, however, it is supposed, were used for ornamenting shields, sword handles, etc.

The first lenses that we can find any really reliable record of are of those of spectacles, and these 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 invented by Salvino d'Armati, a Florentine; but, like some of those who have followed in his footsteps in the present day, he desired to make his fortune by this invention or discovery by keeping the same secret; but the patent laws, unfortunately for him, were not quite so well developed as at the present time; and a scientist of Pisa, Alessandro della Spina, having seen some of Armati's spectacles made some for himself, and published the method of manufacture.

The gradual and perhaps accidental deepening of the curves of these lenses produced shorter foci, till, by the accidental placing at some distance apart of a concave and convex lens by some children of a Dutch spectacle maker, the telescope was discovered, and from this origin all lenses of the present day have been evolved. All lenses are made of crown or flint glass, the former being free from and the latter containing lead, being slightly more refractive

than the former. The sectional forms of the various lenses are here given:

1 2 3 4 5 6 7 8 9


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