THE MOST POWERFUL TELESCOPE. 339 



test, the giant telescope of Lord Rosse breaks down. It has not the 

 accuracy of definition which constitutes the real power of a telescope, 

 for it is mainly upon this that depends its capability for doing work. 

 Compared w^ith the metal specula w^hich were made at the time when 

 Lord Rosse's telescope was constructed, the great speculum of Lord 

 Rosse's instrument might come out with credit. But great improve- 

 ments have since then been introduced into the manufacture of re- 

 flecting telescopes, and the present silver-on-glass reflecting telescopes 

 successfully rival the finest achromatic telescope in definition and in 

 power. 



In days gone by repeated reference was made to the wonderful 

 things w^hich could be seen upon the surface of the moon with these 

 two giant telescopes of Lord Rosse's. Picturesque descriptions were 

 given of the minute features which were visible ; amazement was often 

 expressed at the small objects which could be seen. Still more inter- 

 esting accounts were given of what ought to be visible — a carpet of 

 pronounced pattern as big as Lincoln's Inn Fields, the Castle at Dub- 

 lin, the Court-house at Cork, a house, or even a man, provided he were 

 big enough. All these ought to be seen if they happened to be on the 

 lower surface. Yet when we come to consider what it really is w^hich 

 is described as being seen, when we calmly examine the various draw- 

 ings which have been made by the aid of one or the other of these 

 great telescopes, then we find that they show nothing which can not 

 be distinctly seen and drawn by the smallest astronomical telescope of 

 high excellence. An enormous blaze of light is gathered by the tele- 

 scopes, but all this light reveals nothing which can not be seen with 

 far greater ease in a far smaller telescope. There are in existence a 

 number of drawings of the planets, and observations of their satellites ; 

 there are also observations of close double stars, or faint companions 

 to bright stars, all made with one or the other of these two telescopes. 

 Yet nothing has been seen which is beyond the power of a good astro- 

 nomical telescope of comparatively moderate aperture. It is only in 

 observing the dull, ill-defined nebulse that Lord Rosse's great telescope 

 has any exceptional advantage, though even in this respect it is prob- 

 ably much overrated. As an astronomical telescope either of Lord 

 Rosse's telescopes would be fairly beaten by either of the fine eighteen- 

 inch reflectors which are now in existence. 



If, then, Lord Rosse's great telescope is not the most powerful in 

 existence, what answer is to be given to the question with which Ave 

 commenced ? Which is the most powerful telescope in existence ? 

 There are the great refractors of Poltava and of Cambridge, United 

 States, each of fifteen inches in diameter and twenty-three feet in focal 

 length. There is the still larger refractor of Chicago, with an aperture 

 of eighteen inches and a focal length of twenty-three feet. All these 

 instruments are of high excellence in defining power, the essential point 

 where Lord Rosse's breaks down. There is the reflector of Mr. Las- 



