Page:Astronomy for Everybody.djvu/94

great instrument in dimensions. The largest mirrors so far successfully made and used have been about four feet in diameter. About fifty years ago, Mr. Lassell made one of this size, with which he discovered two new satellites of Uranus. More recently, Mr. A. A. Common, F.R.S., has constructed a mirror of the same size. This has been used in taking photographs of nebulæ and other faint objects, for which this form of telescope seems well designed.

The great difficulty in using a large mirror is that it bends under the influence of its own weight. It would seem that when the diameter exceeds four feet, no way of completely avoiding this difficulty has yet been put into successful use. A mirror of five feet diameter is, however, being made at the Yerkes Observatory by Mr. Ritchie, in which, it is hoped, all the difficulties will be surmounted.

In the instruments of Lord Rosse and Mr. Lassell, the mirror was made of an alloy, known as speculum metal. Recently, however, the use of speculum metal has been superseded by another arrangement. The concave mirror is made of a large disk of glass, which is ground and polished into nearly spherical form, or to speak more accurately, a parabolic form, because the latter is necessary to bring all the rays to one focus. A thin coating of silver is then deposited on the surface of the glass, which is susceptible of a high polish, and reflects more light than polished metal.