Page:Popular Science Monthly Volume 16.djvu/359

Rh test, the giant telescope of Lord Rosse breaks down. It has not the accuracy of definition which constitutes the real power of a telescope, for it is mainly upon this that depends its capability for doing work. Compared with the metal specula which were made at the time when Lord Rosse's telescope was constructed, the great speculum of Lord Rosse's instrument might come out with credit. But great improvements have since then been introduced into the manufacture of reflecting telescopes, and the present silver-on-glass reflecting telescopes successfully rival the finest achromatic telescope in definition and in power.

In days gone by repeated reference was made to the wonderful things which could be seen upon the surface of the moon with these two giant telescopes of Lord Rosse's. Picturesque descriptions were given of the minute features which were visible; amazement was often expressed at the small objects which could be seen. Still more interesting accounts were given of what ought to be visible—a carpet of pronounced pattern as big as Lincoln's Inn Fields, the Castle at Dublin, the Court-house at Cork, a house, or even a man, provided he were big enough. All these ought to be seen if they happened to be on the lower surface. Yet when we come to consider what it really is which is described as being seen, when we calmly examine the various drawings which have been made by the aid of one or the other of these great telescopes, then we find that they show nothing which can not be distinctly seen and drawn by the smallest astronomical telescope of high excellence. An enormous blaze of light is gathered by the telescopes, but all this light reveals nothing which can not be seen with far greater ease in a far smaller telescope. There are in existence a number of drawings of the planets, and observations of their satellites; there are also observations of close double stars, or faint companions to bright stars, all made with one or the other of these two telescopes. Yet nothing has been seen which is beyond the power of a good astronomical telescope of comparatively moderate aperture. It is only in observing the dull, ill-defined nebulæ that Lord Rosse's great telescope has any exceptional advantage, though even in this respect it is probably much overrated. As an astronomical telescope either of Lord Rosse's telescopes would be fairly beaten by either of the fine eighteen inch reflectors which are now in existence.

If, then, Lord Rosse's great telescope is not the most powerful in existence, what answer is to be given to the question with which we commenced? Which is the most powerful telescope in existence? There are the great refractors of Poltava and of Cambridge, United States, each of fifteen inches in diameter and twenty-three feet in focal length. There is the still larger refractor of Chicago, with an aperture of eighteen inches and a focal length of twenty-three feet. All these instruments are of high excellence in defining power, the essential point where Lord Rosse's breaks down. There is the reflector of Mr.