h prolonged fatigue. Nothing in the world would
induce Herschel to abandon his work; for, according to him, it would be
to spoil it."

The advantages that Herschel found in 1783, 1784, 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 instrument 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 appear 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[18] 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 produced by means of a lenticular glass, the place it occupies 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 examine that image, must necessarily place himself _beyond_
the point where the rays that form it have crossed each other; _beyond_,
let us carefully remark, means _farther off_ from the object-glass. The
observer's head, his body, cannot then injure the formation or the
brightness of the image, however small may be the distance from which we
have to study it. But it is no longer thus with the image formed by
means of reflection. For the image is now placed between the object and
the reflecting mirror; and when the astronomer approaches in order to
examine it, he inevitably intercepts, if not the totality, at least a
very considerable portion of the luminous rays, which would otherwise
have c