The Improvement of Telescopes and Optical Apparatus.
It will be of interest to give in this place some connected account of the large forty-foot reflector, of four feet aperture, made by Herschel. Its history extends from 1785 to 1811. Its manufacture was considered by his cotemporaries as his greatest triumph. As a machine, it was extremely ingenious in all its parts, as may be seen from the elaborate description and plates of it published in the Philosophical Transactions for 1795. One of its mirrors certainly had good definition, for, by means of it, the two small satellites of Saturn (Mimas and Enceladus) were discovered, and these discoveries alone would make it famous. Perhaps more was expected of it by the public in general than it absolutely performed. Its merits were after a while decried, and Herschel even felt obliged to state why he did not always employ it in his observations. His reasons were perfectly valid, and such as any one may understand. The time required to get so large a machine
