The New York Independent of February 1874 thus describes the largest refractor in the world :—
The great telescope of the Clarks, recently mounted at the Naval Observatory, in Washington, is probably, on the whole, the most powerful in the world. There are in existence a few reflectors of larger size. We recall at the moment the following—viz. the telescopes of Lord Rosse, one of 6 feet diameter, which is now out of use, and one 3 feet in diameter, with which some good work has been done within a few years; the 4-foot reflector of Mr. Lassell, not now in use; the great 4-foot reflector of the Melbourne Observatory, which is in use, but thus far disappoints expectation ; the 40-inch silvered-glass reflector of the Marseilles Observatory, which is a good and useful instrument in its line of work ; and, finally, the silvered-glass reflector of Dr. Draper, in this city, with an aperture of 28 inches.
These instruments all have a larger aperture than the
Washington telescope, whose diameter is 26 inches. But the latter is a
refractor—i.e. it forms the image of a celestial object by means of a lens,
instead of a mirror; and an instrument of this kind is generally fully a match
for a much larger reflector. The only other refractor in existence which can at
all compete with it is the telescope made by Cooke, of England, and belonging
to Mr. Newhall, of Newcastle-on-Tyne, which has an aperture of 25 inches and a
focal length of 33 feet. There are one or two other instruments in England
having apertures of 20 and 21 inches; but nothing is heard from them, and they
are probably of inferior quality. Next in size comes the Chicago instrument,
with a diameter of 18 inches; and next to this the great refractors of
Cambridge (U.S.) and Poulkowa, with apertures of 15 inches. The Washington
instrument, as has been said, has an aperture of 26 inches, and its focal
length is 35 feet. It has a steel tube, shaped much like a cigar, and is
mounted upon enormous axes of steel, upon which it turns freely with the
pressure of a single finger. Its elaborate clockwork is driven by a small
waterwheel, and makes it follow the diurnal motion of the stars with perfect
precision.
The building in which it is placed is an admirable
innovation on old-fashioned observatories, being as light and thin as possible
consistent with sufficient strength. The walls are made of vertical oak posts,
set some eight or ten feet apart, covered on the outside with sheet iron
(painted, of course), and on the inside with paper. In very hot or very cold
weather an observatory with walls of brick or masonry accommodates itself to
the change of temperature at night and morning so slowly that for many hours
each day the instruments are rendered almost useless by the currents of heated
air; but in a building of this sort there is no such difficulty. The telescope
sustains perfectly all the tests to which it has been subjected, showing the
highest order of excellence in its materials and workmanship. At present its
principal work is in securing measures of the satellites of Uranus and Neptune,
as Professor Newcomb, who is in charge of it, takes an almost fatherly interest
in those planets, the investigation of whose orbits has been the most important
labour of his scientific life. His recently published work on Uranus has just
been crowned with the gold medal of the Royal Astronomical Society of Great
Britain. No second satellite of Neptune has yet been seen, nor any satellites
of Uranus, except the four observed by Lassell, and known as Ariel, Umbriel,
Titania, and Oberon. These are all well seen and satisfactorily measured.
Somehow or other, a report has crept into the newspapers, much to the annoyance
of the observers, that the telescope shows only two of the Uranian satellites.
Nothing has yet been seen of the companion of Procyon, whose discovery was
announced from Poulkowa. last winter, although it has been carefully looked
for. It seems to be probable to say the least that the Russian observers were
mistaken.
Professor Young, who has just been to see it, writes to our
correspondent in the most enthusiastic terms. “It is glorious” he says, “I mean
the nebula of Orion seen through it”.
www.theramblingastronomer.co.uk
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