are all mechanical and are made with a spherometer. Such tests, however, simply insure accurate curvature, and by their very nature can take no account of irregularities in the texture of the glass. These can only be detected and remedied by means of optical tests. When the preliminary polishing is finished, the lens
Fig. 3.—Grinding Telescope-lenses.
is roughly mounted and submitted to a most rigid examination. A beam of light from what is called at the workshops an "artificial star" is transmitted through the lens and enables the workmen 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
