The plan of this Telescope was intimated by Herschel, through Sir
Joseph Banks, to George III., who offered to defray the whole expense
of it; a noble act of liberality, which has never been imitated by
any other British sovereign. Towards the close of 1785, accordingly,
Herschel began to construct his reflecting telescope, _forty feet in
length_, and having a speculum _fully four feet in diameter_. The
thickness of the speculum, which was uniform in every part, was 3½
inches, and its weight nearly 2118 pounds; the metal being composed of
32 copper, and 10·7 of tin: it was the third speculum cast, the two
previous attempts having failed. The speculum, when not in use, was
preserved from damp by a tin cover, fitted upon a rim of close-grained
cloth. The tube of the telescope was 39 ft. 4 in. long, and its width 4
ft. 10 in.; it was made of iron, and was 3000 lbs. lighter than if it
had been made of wood. The observer was seated in a suspended movable
seat at the mouth of the tube, and viewed the image of the object with
a magnifying lens or eye-piece. The focus of the speculum, or place of
the image, was within four inches of the lower side of the mouth of the
tube, and came forward into the air, so that there was space for part
of the head above the eye, to prevent it from intercepting many of the
rays going from the object to the mirror. The eye-piece moved in a tube
carried by a slider directed to the centre of the speculum, and fixed
on an adjustible foundation at the mouth of the tube. It was completed
on the 27th August 1789; and _the very first moment_ it was directed to
the heavens, a new body was added to the solar system, namely, Saturn
and six of its satellites; and in less than a month after, the seventh
satellite of Saturn, “an object,” says Sir John Herschel, “of a far
higher order of difficulty.”--_Abridged from the North-British Review_,
No. 3.
This magnificent instrument stood on the lawn in the rear of Sir
William Herschel’s house at Slough; and some of our readers, like
ourselves, may remember its extraordinary aspect when seen from
the Bath coach-road, and the road to Windsor. The difficulty of
managing so large an instrument--requiring as it did two assistants
in addition to the observer himself and the person employed to note
the time--prevented its being much used. Sir John Herschel, in a
letter to Mr. Weld, states the entire cost of its construction,
4000_l._, was defrayed by George III. In 1839, the woodwork of
the telescope being decayed, Sir John Herschel had it cleared
away; and piers were erected, on which the tube was placed, _that_
being of iron, and so well preserved that, although not more than
one-twentieth of an inch thick, when in the horizontal position
it contained within all Sir John’s family; and next the two
reflectors, the polishing apparatus, and portions of the machinery,
to the amount of a great many tons. Sir John attributes this great
strength and resistance to the internal structure of the tube, very
similar to that patented under the name of corrugated iron-roping.
Sir John Herschel also thinks that system of triangular arrangement
of the woodwork was upon the principle to which “diagonal bracing”
owes its strength.