Object-Glass, or OBJECTIVE

Lens furthest from the eye in a telescope or microscope. Its function is to form a real image of the object, this image being magnified by the eyepiece. Object-gla es are converging and achromatic: in the case of the telescope they are made very large, to utilize as much light as po ible. The great refractor at the Washington Observatory has an aperture of 26 in., that of the Lick Observatory, Cal., 36 in., and that of the Yerkes Observatory, Chicago Univ., 41 in. These were all made by Alvan Clark & Sons, Cambridge, Ma . The gla plates were imported. The cost of those for the Yerkes lenses was $40,000; of the finished lenses $100,000. The convex lens of crown gla is 3 in. thick at the center. 14 in. at the edge, and weighs 205 lbs.; the concave lens of flint gla weighs 310 lbs.; with the iron ring and cell the total weight is 1,000 lbs.