Sir David Brewster has remarked, that “the long interval of half
a century seems to be the period of hybernation during which the
telescopic mind rests from its labours in order to acquire strength for
some great achievement. Fifty years elapsed between the dwarf telescope
of Newton and the large instruments of Hadley; other fifty years rolled
on before Sir William Herschel constructed his magnificent telescope;
and fifty years more passed away before the Earl of Rosse produced
that colossal instrument which has already achieved such brilliant
discoveries.”[25]
In the improvement of the Reflecting Telescope, the first object
has always been to increase the magnifying power and light by the
construction of as large a mirror as possible; and to this point Lord
Rosse’s attention was directed as early as 1828, the field of operation
being at his lordship’s seat, Birr Castle at Parsonstown, about fifty
miles west of Dublin. For this high branch of scientific inquiry Lord
Rosse was well fitted by a rare combination of “talent to devise,
patience to bear disappointment, perseverance, profound mathematical
knowledge, mechanical skill, and uninterrupted leisure from other
pursuits;”[26] all these, however, would not have been sufficient, had
not a great command of money been added; the gigantic telescope we are
about to describe having cost certainly not less than twelve thousand
pounds.
Lord Rosse ground and polished specula fifteen inches, two feet,
and three feet in diameter before he commenced the colossal
instrument. It is impossible here to detail the admirable
contrivances and processes by which he prepared himself for this
great work. He first ascertained the most useful combination of
metals for specula, both in whiteness, porosity, and hardness,
to be copper and tin. Of this compound the reflector was cast in
pieces, which were fixed on a bed of zinc and copper,--a species
of brass which expanded in the same degree by heat as the pieces
of the speculum themselves. They were ground as one body to a true
surface, and then polished by machinery moved by a steam-engine.
The peculiarities of this mechanism were entirely Lord Rosse’s
invention, and the result of close calculation and observation:
they were chiefly, placing the speculum with the face upward,
regulating the temperature by having it immersed in water, usually
at 55° Fahr., and regulating the pressure and velocity. This was
found to work a perfect spherical figure in large surfaces with
a degree of precision unattainable by the hand; the polisher, by
working above and upon the face of the speculum, being enabled
to examine the operation as it proceeded without removing the
speculum, which, when a ton weight, is no easy matter.
The contrivance for doing this is very beautiful. The machine is
placed in a room at the bottom of a high tower, in the successive
floors of which trap-doors can be opened. A mast is elevated on the
top of the tower, so that its summit is about ninety feet _above_
the speculum. A dial-plate is attached to the top of the mast, and
a small plane speculum and eye-piece, with proper adjustments,
are so placed that the combination becomes a Newtonian telescope,
and the dial-plate the object. The last and most important part
of the process of working the speculum, is to give it a _true
parabolic figure_, that is, such a figure that each portion of it
should reflect the incident ray to the same focus. Lord Rosse’s
operations for this purpose consist--1st, of a stroke of the first
eccentric, which carries the polisher along _one-third_ of the
diameter of the speculum; 2d, a transverse stroke twenty-one times
slower, and equal to 0·27 of the same diameter, measured on the
edge of the tank, or 1·7 beyond the centre of the polisher; 3d, a
rotation of the speculum performed in the same time as thirty-seven
of the first strokes; and 4th, a rotation of the polisher in the
same direction about sixteen times slower. If these rules are
attended to, the machine will give the true parabolic figure to the
speculum, whether it be _six inches_ or _three feet in diameter_.
In the three-feet speculum, the figure is so true with the whole
aperture, that it is thrown out of focus by a motion of less
than the _thirtieth of an inch_, “and even with a single lens of
one-eighth of an inch focus, giving a power of 2592, the dots on a
watch-dial are still in some degree defined.”
Thus was executed the three-feet speculum for the twenty-six-feet
telescope placed upon the lawn at Parsonstown, which, in 1840, showed
with powers up to 1000 and even 1600; and which resolved nebulæ into
stars, and destroyed that symmetry of form in globular nebulæ upon
which was founded the hypothesis of the gradual condensation of
nebulous matter into suns and planets.[27]
Scarcely was this instrument out of Lord Rosse’s hands, when he
resolved to attempt by the same processes to construct another
reflector, with a speculum _six feet_ in diameter and _fifty feet
long_! and this magnificent instrument was completed early in 1845.
The focal length of the speculum is fifty-four feet. It weighs four
tons, and, with its supports, is seven times as heavy as the four-feet
speculum of Sir William Herschel. The speculum is placed in one of
the sides of a cubical wooden box, about eight feet wide, and to the
opposite end of this box is fastened the tube, which is made of deal
staves an inch thick, hooped with iron clamp-rings, like a huge cask.
It carries at its upper end, and in the axis of the tube, a small oval
speculum, six inches in its lesser diameter.
The tube is about 50 feet long and 8 feet in diameter in the middle,
and furnished with diaphragms 6½ feet in aperture. The late Dean of Ely
walked through the tube with an umbrella up.
The telescope is established between two lofty castellated piers 60
feet high, and is raised to different altitudes by a strong chain-cable
attached to the top of the tube. This cable passes over a pulley on
a frame down to a windlass on the ground, which is wrought by two
assistants. To the frame are attached chain-guys fastened to the
counterweights; and the telescope is balanced by these counterweights
suspended by chains, which are fixed to the sides of the tube and pass
over large iron pulleys. The immense mass of matter weighs about twelve
tons.
On the eastern pier is a strong semicircle of cast-iron, with which the
telescope is connected by a racked bar, with friction-rollers attached
to the tube by wheelwork, so that by means of a handle near the
eye-piece, the observer can move the telescope along the bar on either
side of the meridian, to the distance of an hour for an equatorial star.
On the western pier are stairs and galleries. The observing gallery is
moved along a railway by means of wheels and a winch; and the mechanism
for raising the galleries to various altitudes is very ingenious.
Sometimes the galleries, filled with observers, are suspended midway
between the two piers, over a chasm sixty feet deep.
An excellent description of this immense Telescope at Birr Castle will
be found in Mr. Weld’s volume of _Vacation Rambles_.
Sir David Brewster thus eloquently sketches the powers of the telescope
at the close of his able description of the instrument, which we have
in part quoted from his _Life of Sir Isaac Newton_.
We have, in the mornings, walked again and again, and ever with
new delight, along its mystic tube, and at midnight, with its
distinguished architect, pondered over the marvellous sights which
it dis-closes,--the satellites and belts and rings of Saturn,--the
old and new ring, which is advancing with its crest of waters to
the body of the planet,--the rocks, and mountains, and valleys, and
extinct volcanoes of the moon,--the crescent of Venus, with its
mountainous outline,--the systems of double and triple stars,--the
nebulæ and starry clusters of every variety of shape,--and those
spiral nebular formations which baffle human comprehension, and
constitute the greatest achievement in modern discovery.
The Astronomer Royal, Mr. Airy, alludes to the impression made by
the enormous light of the telescope,--partly by the modifications
produced in the appearance of nebulæ already figured, partly by the
great number of stars seen at a distance from the Milky Way, and
partly from the prodigious brilliancy of Saturn. The account given by
another astronomer of the appearance of Jupiter was that it resembled a
coach-lamp in the telescope; and this well expresses the blaze of light
which is seen in the instrument.
The Rev. Dr. Scoresby thus records the results of his visits:
The range opened to us by the great telescope at Birr Castle is
best, perhaps, apprehended by the now usual measurement--not of
distances in miles, or millions of miles, or diameters of the
earth’s orbit, but--of the progress of light in free space. The
determination within, no doubt, a small proportion of error of
the parallax of a considerable number of the fixed stars yields,
according to Mr. Peters, a space betwixt us and the fixed stars of
the smallest magnitude, the sixth, ordinarily visible to the naked
eye, of 130 years in the flight of light. This information enables
us, on the principles of _sounding the heavens_, suggested by Sir
W. Herschel, with the photometrical researches on the stars of Dr.
Wollaston and others, to carry the estimation of distances, and
that by no means on vague assumption, to the limits of space opened
out by the most effective telescopes. And from the guidance thus
afforded us as to the comparative power of the six feet speculum
in the penetration of space as already elucidated, we might fairly
assume the fact, that if any other telescope now in use could
follow the sun if removed to the remotest visible position, or
till its light would require 10,000 years to reach us, the grand
instrument at Parsonstown would follow it so far that from 20,000
to 25,000 years would be spent in the transmission of its light to
the earth. But in the cases of clusters of stars, and of nebulæ
exhibiting a mere speck of misty luminosity, from the combined
light of perhaps hundreds of thousands of suns, the _penetration_
into space, compared with the results of ordinary vision, must
be enormous; so that it would not be difficult to show the
_probability_ that a million of years, in flight of light, would
be requisite, in regard to the most distant, to trace the enormous
interval.