work, and learned to polish mirrors like an ordinary workman, the consequence of which was that he could bestow unusual pains upon the finishing of the speculum. His Lordship not only learnt the mere handicraft of speculum polishing, but went deeply into the engineering difficulties of the operation, and succeeded in inventing many improvements for diminishing labour and rendering the form of the surface more perfect. The specula ground and polished under Lord Rosse's method are almost entirely free from what is called spherical aberration,—that is to say, all rays proceeding from a single point of light, such as a star, are collected into a single point instead of being scattered in a round mass. This freedom from spherical aberration is of course necessary to produce perfectly distinct images. In his Life of Newton Sir David Brewster calls it one of the most marvellous combinations of art and science yet seen in the world.
The tube of Lord Rosse's instrument is 55 feet long, and weighs 6-1/2 tons. In form it may be compared to the chimney of a steamboat of enormous size. At one end it terminates in a kind of square box, within which is contained the mirror, whose diameter is 6 feet, and which weighs nearly 4 tons. The weight of the whole apparatus is consequently nearly 10-1/2 tons, or four times as much as Herschel's. It is erected on an oblong mass of masonry, 75 feet in length from north to south, between two solid walls nearly 50 feet high, which serve as supports for the mechanism intended to move this enormous tube in all directions. To the walls are also fixed movable staircases with platforms that can be brought up to the eye-piece with the greatest facility, no matter in what position the telescope may be placed. This noble instrument has penetrated space to a distance perfectly unattempted before its existence, and has resolved numerous nebulæ into masses
