Telescopes of four inches aperture and upward are usually provided with a smaller companion called a _finder_. This has a larger field of view than the main telescope, so that objects which are of sufficient brightness can readily be picked up and brought to the centre of the finder, the adjustments being such that the object is then also at the centre of the field of the large telescope. There are, of course, many practical details connected with the working of an equatorial with which space does not permit us to deal. It may be remarked, however, that the adjustment of the polar axis is very simply performed by first inclining it at an angle approximately equal to the latitude of the place where it is set up, and setting it as nearly as possible in the meridian by means of a compass or by observations of the sun at noon. The final adjustment is then made by a series of observations of stars of known position. SOME OF THE WORLD’S GREAT TELESCOPES.—Thanks to the wide public interest taken in astronomical matters, a large number of powerful telescopes have been set up in various parts of the world. To the British Islands belongs the honor of possessing the largest telescope in the world. This is the giant reflector erected by Lord Rosse, in 1842, at Parsonstown, the mirror being six feet in diameter, and the focal length sixty feet. Many very valuable observations were made with this instrument in its early days, but of late years it seems to have fallen into disuse. One reason may be that the mounting is not of the most convenient form, and makes the telescope unsuitable for photographic work. Coming next in point of size to the Rosse telescope is the reflector erected at Ealing by Dr. A. A. Common. The glass mirror of this telescope is five feet in diameter, five inches thick, and weighs more than half a ton. Dr. Common aimed specially at constructing the largest possible telescope which could be equatorially mounted and provided with a driving-clock, and he was only limited to an aperture of five feet by the impossibility of obtaining a glass disk of larger size. He has attained such great skill in this work that he was able to produce a perfect mirror five feet in diameter in three months’ time, although no less than 410,000 strokes of the polishing machine were required. The telescope is of the Newtonian form, and the mounting is quite unique. The polar axis consists of an iron cylinder, made up of boiler plate, seven feet eight inches in diameter, and about fifteen feet long. From the top of the cylinder, near its outer edge, two horns, each six feet long, project outward, and the tube of the telescope swings on trunnions attached to the ends of the horns. The main part of the telescope tube is square, built up of steel angle iron, and carries the mirror at its lower end; the upper part of the tube, which carries the “flat” and eye-piece, is round, and of tinned steel strengthened by a skeleton framework.