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THE MOST POWERFUL TELESCOPE IN EXISTENCE. 
Br E. NEISON, F.R.A.S. 
W HICH is the most powerful telescope in existence ? De- 
fine the meaning which ought to be attached to the 
adjective 66 powerful” in this question. The most powerful 
telescope in existence is that existing telescope which can do 
the most work. The work of a telescope may be said to be to 
enable you to see and to enable you to measure. Therefore, that 
telescope with which you can see most and can measure best, 
is that which can do the most work, and is unquestionably the 
most powerful telescope in existence. 
Which is the most powerful telescope in existence ? 
Everyone has heard of the two giant telescopes which were 
constructed nearly forty years ago by the late Lord Posse, and 
which were erected at his residence at Parsonstown, about fifty 
miles from Dublin. The first great telescope constructed by 
Lord Rosse was a reflecting telescope with a speculum three feet 
in diameter and twenty-six feet in focal length. It was carried 
in a ponderous tube moving in a massive iron mounting by means 
of ingenious machinery. When it was finished in the year 1 840 
it was considered the grandest instrument in existence, and from 
its employment in the study of the heavens enormous advan- 
tages 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 Posse’s Telescope.” 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 
NEW SERIES, VOL. III. NO. XII. C C 
