21333

Updated: July 2, 2026

Sir John Herschel tells us that "the total thickness to be abraded from the edge of a spherical speculum 48 inches in diameter and 40 feet focus, to convert it into a paraboloid, is only 1/21333 of an inch;" yet upon this minute difference of form depends the clearness of the image, and, as a consequence, the entire efficiency of the instrument.

Agnes M. (Agnes Mary) Clerke - A Popular History of Astronomy During the Nineteenth Century: Fourth Edition