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 for use, not for show, and there is no attempt to make any tool finer than it need be to meet its purpose. But everything is exactly adapted to its purpose. Almost every stage of telescope making may be seen here usually, from the rough discs to the finished instrument; though at any particular time, some stages of progress may not be exemplified, for telescopes are not turned off by thousands, like boots and shoes. The discs for the lenses come from the manufacturers in flat, square plates of such thickness and diameter as may be needed for telescopes of given sizes. The surface is not polished, save at two places on the edges, through which one can look into the glass and out across its greatest diameter. So perfect is the clearness, that a thickness of several inches does not seem to obstruct the sight any more than so much air. Mr. Clark, however, does not accept it as perfect because of this apparent clearness. Every disc of optical glass is subjected to numerous tests before being pronounced suitable for use. Flaws that could not be discovered by the eye are searched for with ingenious instruments. Very large discs are not sent in this rough, square form, but are rounded and polished over the whole surface. These come very carefully packed, as well they may, for the discs alone, before they have been touched by Mr. Clark, are, when large, worth thousands of dollars. After testing the purity of the glass, the first process is the grinding. This is begun with very coarse stone and sand until an approximately correct shape is reached. Then emery of finer and finer quality is used, until the shape is as perfect as it can be made by computation. Then the polishing is quickly done. and the lens is apparently
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