338 THE POPULAR SCIENCE MONTHLY. 



it was iinished in the year 1840 it was considered the grandest instru- 

 ment in existence, and from its emj^loyment in the study of the heav- 

 ens enormous advantages were expected to be gained for astronomy. 

 Scarcely, however, was this telescope out of the hands of its maker, 

 than Lord Rosse resolved to construct a second telescope of still larger 

 dimensions. With enormous skill, patience, and ingenuity Lord Rosse 

 carried out this intention, and by the year 1846 had finished his second 

 grand telescope, the instrument commonly known as " Lord Rosse's Tel- 

 escope." It has a metal speculum six feet in diameter and fifty-four 

 feet in focal length. This enormous mirror, which weighs nearly four 

 tons, is placed in a great tube eight feet in diameter and fifty feet in 

 length, and this tube is carried by a massive iron mounting supported 

 by two lofty castellated buildings, each nearly sixty feet in height. 

 The weight of the telescope and its mountings is enormous. By in- 

 genious methods the observer who is using the telescope is placed in a 

 kind of cage, suspended in the air from the mounting of the telescope, 

 and carried up and down along with the instrument. 



To this day this giant telescope of Lord Rosse's retains its position 

 as the greatest telescope in existence. In its enormous size it has still 

 no rival, in its massiveness and weight it is long likely to retain its 

 preeminence. 



Which is the most powerful telescope in existence ? 



Lord Rosse's giant telescope, of course, will be the answer of most 

 people ; it will be the answer of the great majority of scientific men ; 

 it would be almost the unanimous answer of the British Association, 

 of that Section A which is supposed to keep the world informed of the 

 great achievements of astronomy and of optics. 



Is this the true answer ? No. 



To most people, to most scientific men, this answer will come like 

 a shock, for to them it has long been a cherished tradition, an article 

 of faith, almost an axiom, that Lord Rosse's giant telescope was the 

 most powerful telescope in existence. To those astronomers who ai'e 

 observers, astronomers not star-gazers, it is well known that for years 

 this giant telescope of Lord Rosse's has been beaten in power by far 

 smaller and more compact rivals. In fact, it is doubtful whether in 

 real power it is much superior to its smaller companion, the three-foot 

 telescope. 



There are many who judge a telescope by its size alone, who com- 

 pute its excellence by aid of a two-foot rule, and a knowledge of its 

 cost in pounds. With them a telescope with a metallic speculum 

 weighing four tons and measuring six feet in diameter, with a tube 

 fifty feet long, and costing a thousand pounds, ought to give so much 

 light, have such and such separating power, and show this or that ob- 

 ject. It is true with small telescopes a great deal may be done in this 

 way, but experienced observers know that the real power of a telescope 

 •can only be ascertained by a study of what it has done. Tried by this 



