388 
POPULAR SCIENCE REVIEW. 
There is also the great reflector of the Paris Observatory, with 
a silver-on- glass speculum nearly four feet in diameter, an in- 
strument 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 Grates- 
head, 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 33 feet in focal length. This magni- 
ficent instrument is equatorially mounted and driven by clock- 
work, 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 New- 
comb, 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, 37-J 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 
which is in existence. 
For the last three years Mr. Common has had in his obser- 
vatory a fine silver-on-glass reflector with an aperture of eighteen 
inches and a focal length of nearly ten feet. This telescope 
was mounted by him on an equatorial stand of his own design, 
on what is known as the “ Sissons ” principle. For efficiency, 
power, and excellence this eighteen-inch reflector is as yet without 
