340 THE POPULAR SCIENCE MONTHLY, 



sells, 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 lai'ge refrac- 

 tor 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 excel- 

 lence. 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, Hol- 

 den, 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 com- 

 menced ought to be — the great refractor of the Washington Observa- 

 tory. 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 speculuiu, 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 efiicacious 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 



