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 aérial 
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.— 77anslator’s 
Note. 
\) =a 
