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THE POPULAR SCIENCE MONTHLY. 



are all mechanical and are made with a spherometer. Such tests, 

 however, simply insure accurate curvature, and by their very na- 

 ture can take no account of irregularities in the texture of the 

 glass. These can only be detected and remedied by means of opti- 

 cal tests. When the preliminary polishing is finished, the lens 



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is roughly mounted and submitted to a most rigid examination. 

 A beam of light from what is called at the workshops an " artifi- 

 cial star " is transmitted through the lens and enables the work- 

 men to locate defects of all sorts. The remedy is then a matter 

 of touch and try, and, as one can readily imagine, is a long and 

 tedious process. Still, the lens is not completed. It must now be 

 submitted to the test of actual star-gazing. The most famous 

 lens turned out by the Messrs. Clark, and indeed the largest in 



