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[153] finished. But it is only begun. The larger part of the work is yet to be done. A perfect object glass cannot be made by theory; it must be tested over and over again. The first tests show the principal defects, which are remedied as they are discovered. Then the process becomes increasingly delicate. Every part of the lens is tested by ingenious devices, and minute corrections are made. After months of this sort of work, the glass may be tried out of doors in a temporary frame. Then more corrections follow. As long as Mr. Clark has a telescope in the shop he feels he can do something to improve it. At this writing, the object glasses for the Yerkes telescope have been practically done for more than a year, yet Mr. Clark expects to keep them for several months still, making final minute corrections. Of course a small lens can be made in a much shorter time.

The nervous strain of making a large object glass is severe. Mr. Clark is not sure that he would undertake to exceed the forty-inch glass, if he should be asked. He certainly would refuse unless allowed a voice in the selection of the place where it should be mounted. The finest of glasses would be of little use, unless suitably mounted. Indeed, the larger and finer the glass, the more imperative is a good mounting. So the manufacture of mountings is a very important part of Mr. Clark's business. A large space is given up to this, and one soon discovers that in its way the work is as delicate as the grinding of the lenses. No ordinary machinery or labor can be employed. Microscopical accuracy must be observed in every part.

After looking at the different processes in the manufacture of a telescope, the visitor may be fortunate enough to see a complete telescope mounted

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George B. Clark (4)
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