Page:Popular Science Monthly Volume 16.djvu/360

340, with its metal speculum of two feet in diameter and its tube twenty feet in length. There is the great Melbourne reflector, with its great metal speculum of forty-eight inches in diameter, the second largest telescope in the world, but by no means so sharp in definition as might be desired, so that it failed to reveal the satellites of Mars which were seen with an instrument of not one sixth the diameter in Europe.

There is also the great reflector of the Paris Observatory, with a silver-on-glass speculum nearly four feet in diameter, an instrument whose power is seriously injured by the imperfect definition arising from the flexure of its thin speculum. There is also the large refractor constructed for Mr. Newall, of Gateshead, with an object-glass twenty-five inches in diameter mounted in a tube nearly thirty feet in length.

But all these instruments must yield the palm to the great refractor of the United States Naval Observatory at Washington, a splendid instrument, with an object-glass twenty-six inches in clear aperture and thirty-three feet in focal length. This magnificent instrument is equatorially mounted and driven by clockwork, so that it is complete as an astronomical telescope. The Washington refractor is, however, not merely a telescope of great dimensions; like more than one of those previously mentioned, it is an instrument of high optical excellence. Its definition is crisp and sharp, and it brings every ray of the enormous amount of light which it collects to a sharp focus as a very minute point, so that none is wasted. It was with this fine telescope that Professor Asaph Hall made his famous discovery of the satellites of Mars, that Mr. Burnham discovered a number of the most minute companions to the brighter stars, and that Professors Newcomb, Holden, and Hall have observed and measured the smallest satellites of Saturn, Uranus, and Neptune. It is this magnificent instrument which is supposed by most astronomers to be the most powerful telescope in existence. Then our answer to the question with which we have commenced ought to be—the great refractor of the Washington Observatory. No!

Then which is the most powerful telescope in existence?

The most powerful telescope in existence is the magnificent new reflecting telescope which has been just finished by Mr. A. Ainslie Common, and is erected at his residence at Ealing. This telescope has a silver-on-glass speculum, thirty-seven and a half inches in diameter, and a focal length of just over twenty feet. It is equatorially mounted in a novel but most efficacious manner, and is driven by a powerful clock controlled in an ingenious manner by a method invented by Mr. Common. This new telescope, which has only been finished about a month, has turned out a great success, and is unquestionably the finest and most powerful telescope in existence.

For the last three years Mr. Common has had in his observatory a