A spherometer is used primarily to measure the curvature of objects such as lenses and curved mirrors. This example came from the U.S. Naval Observatory. It was made by Alvan Clark & Sons for the U.S. expeditions to observe the 1874 transit of Venus. It has only a horizontal scale and no vertical scale common to most spherometers. The box for this sphereometer appears to be homemade.
Alvan Clark (1804-1887) was the first prominent American telescope maker. In 1846 he went into business with his two sons, Alvan G. and George B. Clark, founding Clark & Sons. At first they repaired scientific instruments, but the firm quickly progressed on to making telescopes. During the second half of the 19th century, five of Clarks' telescopes held the record for world's largest. To aid in grinding lenses, Clark & Sons also made devices such as spherometers.
Ref: Warner, D. J., "Alvan Clark & Sons: Artists in Optics," United States National Museum Bulletin 274 (1968): 1-13.
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