William Herschel's Contributions

Despite the attendant difficulties, a number of very large specula were made, some of the best by William Herschel. Born in Hanover, Germany, Herschel settled in England in 1757, where he became interested in astronomy and later (1776) turned his attention to telescopes. Working entirely by hand, at first as an amateur, he practiced and developed his technique on a great number of Newtonian telescopes, and learned how to figure the mirrors far better than had any of his predecessors.

He performed the polishing in the conventional manner, with the mirror on top, and used a sweeping, circular stroke for parabolizing. Later, Herschel applied himself to the design now referred to as the Herschelian type, which had been proposed by LeMaire, a French scientist, in 1728.

In this design (Fig. 12), the mirror is tilted so that the image is thrown to one side of the open end of the tube, where it can be examined in comfort, with the observer's back to the object, and without the introduction of a second reflection. This latter feature was of tremendous importance in the days of speculum, when 40 per cent of the light was absorbed in undergoing a single reflection. Of less importance, but nonetheless gainful, was the elimination of the harmful diffraction effects from the secondary mirror.

William Herschel continued