272 HERSCHEL. 



and 1785, in employing telescopes of twenty feet and 

 with large apertures, made him wish to construct much 

 larger still. The expense would be considerable ; King 

 George III. provided for it. The work, begun about the 

 close of 1785, was finished in August, 1789. This in- 

 strument had an iron cylindrical tube, thirty-nine feet 

 four inches English in length, and four feet ten inches 

 in diameter. Such dimensions are enormous compared 

 with those of telescopes made till then. They will ap- 

 pear but small, however, to persons who have heard the 

 report of a pretended ball given in the Slough telescope. 

 The propagators of this popular rumour had confounded 

 the astronomer Herschel with the brewer Meux, and a 

 cylinder in which a man of the smallest stature could 

 scarcely stand upright, with certain wooden vats, as large 

 as a house, in which beer is made and kept in London. 



Herschel's telescope, forty English feet* in length, 

 allowed of the realization of an idea, the advantages of 

 which would not be sufficiently appreciated if I did not 

 here recall to mind some facts. 



In any telescope, whether refracting or reflecting, there 

 are two principal parts: the part that forms the aerial 

 images of the distant objects, and the small lens by the 

 aid of which these images are enlarged just as if they 

 consisted of radiating matter. When the image is pro- 

 duced by means of a lenticular glass, the place it occu- 

 pies will be found in the prolongation of the line that 

 extends from the object to the centre of the lens. The 

 astronomer, furnished with an eye-piece, and wishing to 



* Conforming to general usage, and to Sir W. Herschel himself, we 

 shall allude to this instrument as the forty-foot telescope, though M. 

 Arago adheres to thirty-nine feet and drops the inches, probably be- 

 cause the Parisian foot is rather longer than the English. Translator's 

 Note. 



