The Dictionary Of Photography

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

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Lens
being waste. The pieces are then softened in a muffle furnace, and formed into plates about two or two and a half inches thick. Sometimes the plates are then pressed, after being softened by heat, into rough moulds of clay or iron coated with sand so as to give them a rough form; but the best opticians prefer grinding, as striae are not so liable to be formed. The rough-shaped glasses have now to be made into perfect lenses, for which purpose extreme care is necessary, approximate forms being given by grinding with emery in concave or convex tools of cast iron. It is in the following operations that the greatest skill and care of the optician are required: - The roughly shaped lens is now to be ground with emery in spherical tools of brass or iron. These are given the necessary curves by means of accurate gauges. The roughly fashioned glass or lens is fixed to a plate of brass by means of pitch, and is then worked in the tool with rough emery moistened with water; when the glass is found to touch the tool at all points, finer emery is used, and it is worked a little more, the gauge being now frequently applied to the tool, to see that the radii of curvatures are not altered; then finer kinds of emery still are used, till at last some degree of polish begins to show ; fine putty powder Hs then substituted for emery, and the polishing is commenced. The operation of polishing is Veally the test of a good optician, as this process may alter the sphericity of the lens to such a degree as to completely alter the character of the lens. The lens is fixed on to a block of wood by means of a pitchy cement, and a tool is coated with a resinous mixture, and fine rouge is sprinkled on the tool when cold land the polishing finished entirely by hand. When two lenses are required to be cemented together so as to present one common surface, they are slightly warmed, and a drop or two of Canada balsam is applied, and the two lenses pressed forcibly together, so as to squeeze out excess of balsam. When cooled, they present the appearance of one single piece of glass, and cannot be separated without heat. When two lenses have not a common surface, three small pieces of tinfoil are introduced at equal distances apart between their margins, or • when the separation is greater, as in most portrait lenses, a ring of brass is used for the same purpose. When the lens is fixed in its brass ring, so that it cannot be taken out without raising the bent edge of the brass, it is said to be set. Under the
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