with circles which enable the telescope to be pointed to any desired
object of known right ascension and declination. One of these is the
declination circle, attached to the declination axis and read by a
vernier fixed to the sleeve in which the axis turns; this is adjusted
so as to read 0° when the telescope points to any part of the
celestial equator, and 90° when it is directed to the pole. The other
circle is attached to the polar axis, and determines the position of
the telescope with regard to the meridian; this is called the _hour
circle_, and is divided into twenty-four hours. When the telescope
is on the meridian, the hour circle reads zero, so that its reading
in any other position gives the hour angle of the telescope. Having
given the right ascension and declination of a heavenly body which it
is desired to observe, the telescope is turned until the declination
circle reads the proper angle, and the hour circle indicates the
hour angle which is calculated for the particular moment of pointing
the telescope. [The hour angle is the difference between the right
ascension of the object and the sidereal time of observation.] In
this way it is easy to find objects of known position which are
invisible to the naked eye, and one can even pick up the planets and
brighter stars in full sunshine. Conversely, one can determine from
the circles the right ascension and declination of any object under
observation, but for various reasons only approximate results can be
obtained in this way. The chief use of the circles on an equatorial
is therefore to provide a means of pointing the telescope.
Telescopes of four inches aperture and upward are usually provided
with a smaller companion called a _finder_. This has a larger field
of view than the main telescope, so that objects which are of
sufficient brightness can readily be picked up and brought to the
centre of the finder, the adjustments being such that the object is
then also at the centre of the field of the large telescope.
There are, of course, many practical details connected with the
working of an equatorial with which space does not permit us to deal.
It may be remarked, however, that the adjustment of the polar axis is
very simply performed by first inclining it at an angle approximately
equal to the latitude of the place where it is set up, and setting
it as nearly as possible in the meridian by means of a compass or by
observations of the sun at noon. The final adjustment is then made by
a series of observations of stars of known position.
SOME OF THE WORLD’S GREAT TELESCOPES.—Thanks to the wide public
interest taken in astronomical matters, a large number of powerful
telescopes have been set up in various parts of the world. To the
British Islands belongs the honor of possessing the largest telescope
in the world. This is the giant reflector erected by Lord Rosse, in
1842, at Parsonstown, the mirror being six feet in diameter, and the
focal length sixty feet. Many very valuable observations were made
with this instrument in its early days, but of late years it seems to
have fallen into disuse. One reason may be that the mounting is not
of the most convenient form, and makes the telescope unsuitable for
photographic work.
Coming next in point of size to the Rosse telescope is the reflector
erected at Ealing by Dr. A. A. Common. The glass mirror of this
telescope is five feet in diameter, five inches thick, and weighs
more than half a ton. Dr. Common aimed specially at constructing the
largest possible telescope which could be equatorially mounted and
provided with a driving-clock, and he was only limited to an aperture
of five feet by the impossibility of obtaining a glass disk of larger
size. He has attained such great skill in this work that he was able
to produce a perfect mirror five feet in diameter in three months’
time, although no less than 410,000 strokes of the polishing machine
were required.
The telescope is of the Newtonian form, and the mounting is quite
unique. The polar axis consists of an iron cylinder, made up of
boiler plate, seven feet eight inches in diameter, and about fifteen
feet long. From the top of the cylinder, near its outer edge, two
horns, each six feet long, project outward, and the tube of the
telescope swings on trunnions attached to the ends of the horns. The
main part of the telescope tube is square, built up of steel angle
iron, and carries the mirror at its lower end; the upper part of the
tube, which carries the “flat” and eye-piece, is round, and of tinned
steel strengthened by a skeleton framework.
It is evident that such an enormous instrument as this can not be
made to travel by clockwork with the necessary uniformity without
some very efficient arrangement for reducing friction. Dr. Common’s
plan—and it is here that his instrument is unlike others—is to make
the hollow polar axis watertight, and to fix it in a tank of water.
At the bottom of the polar axis is a ball and socket joint to keep
it in position, and at the top is another bearing, which can be
adjusted so that the polar axis lies truly in the meridian. It was
found necessary to introduce nine tons of iron into the bottom of
the hollow polar axis in order to sink it to the proper angle, and
to put sufficient weight on the bearings to give stability to the
instrument. In this way the great mass is brought into the region of
manageability, and the driving-clock, which is driven by a weight of
one and a half tons, is able to do its work efficiently. Such, in
general outline, is this wonderful telescope, which, although not so
large as Lord Rosse’s famous instrument, is undoubtedly its superior
in light-grasping power and general utility, and more especially in
its adaptability for photographing the heavens.
Among other large reflecting telescopes now in use are the 4-foot
reflectors at Melbourne and Paris, and the 3-foot reflectors at South
Kensington and the Lick Observatory, California.
The largest refracting telescope yet constructed is one of forty
inches aperture for the Yerkes Observatory of the University of
Chicago. It is interesting to note here that Professor Keeler, in
his report as an expert upon the performance of the object-glass,
considers that there is “evidence for the first time that we
are approaching the limit of size in the construction of great
objectives.” Unlike a mirror, a lens can be supported only upon its
circumference, and it is the bending by its own weight that proves
detrimental to its defining power. If the lens be made thicker with a
view of overcoming this defect, the absorption of light by the glass
increases, so that there is in the end no special gain by increasing
the size.
The length of the Yerkes telescope is 62 feet, and is provided
with all accessories pertaining to astrophysical research. The
world-renowed Lick Telescope is of thirty-six inches aperture. The
story of the foundation of this monster instrument is not much
less wonderful than the telescope itself. Brought up in poor
circumstances, with few opportunities for intellectual development,
James Lick, nevertheless, amassed a fortune in business, and having
few relations, he was anxious to dispose of his wealth in such a way
as to bring him that fame which he had failed to achieve in other
directions. Although it is very probable that he had never looked
through a telescope in his life, the idea of a large telescope had
taken a very firm hold upon his mind, and, thanks to the influence of
his advisers, it was definitely announced in 1873 that Mr. Lick’s bid
for immortality was to take this form. Several sites were examined
by experts, and finally Mount Hamilton, California, 4,200 feet above
sea-level, was selected. An excellent road, twenty-six miles in
length, made at the cost of the county authorities, connects the
observatory with the nearest town, San José, thirteen miles distant.
Owing to various delays, operations were not commenced until 1880,
and five years were consumed in clearing away 72,000 tons of rocks
and in erecting the buildings.
Mr. Lick had stipulated for the erection of “a telescope superior to
and more powerful than any telescope yet made,” and Messrs. Alvan
Clark & Co. contracted to supply a lens of thirty-six inches aperture
for the sum of $50,000. It turned out, however, that it was much
easier to make such a contract than to fulfil it. To produce large
disks of optically perfect glass, even in the rough, requires the
greatest possible skill and patience, and this part of the work was
undertaken by Feil & Co. of Paris. The flint glass disk was safely
delivered in America in 1882, but the crown disk was cracked in
packing. The elder Feil having retired from business, the duty of
providing a new block of crown glass devolved upon his sons, who,
after two years spent in vain attempts, ended in bankruptcy, and it
was only through the elder Feil again resuming business that the
much-required disk was finally completed in 1885. After the lapse of
another year, the rough disks were fashioned, in the workshops of the
Clarks, into the most marvelous of telescopic lenses.
The mounting of the object-glass is worthy of the occasion. The tube
is no less than thirty-seven feet long, and four feet in diameter
in the middle part. An iron pier, thirty-eight feet high, beneath
which lie the remains of Mr. Lick, supports the equatorial head,
and a winding staircase enables the observer to reach the setting
circles. Inside the hollow pier is the powerful driving-clock which
turns the telescope to follow the heavenly bodies in their apparent
movements. Finders of six, four, and three inches diameter, rods for
the manipulation of the instrument, and all necessary accessories,
complete what must long remain one of the most perfect instruments at
the service of astronomical science. The $200,000 expended upon it
have already been amply justified by the work accomplished, while Mr.
Lick’s dream of immortality has become a reality.
The following list indicates some of the large refractors now doing
active service:
Aperture Observatory
36 inch [Lick] California.
30 ” Pulkowa, Russia.
30 ” [Bischoffeim] Nice.
28 ” Greenwich.
27 ” Vienna.
26 ” Washington.
25 ” [Newall] Cambridge.
24 ” [Lowell] Mexico.
23 ” Princeton, New Jersey.
It is right to add, however, that opinion is still greatly divided
as to whether these telescopes of large aperture really repay
the expense and labor involved in their erection and use. On the
very rare occasion when the “seeing” is practically perfect—which
occurs perhaps only a few hours in a year—it is probable that the
superiority of a large telescope is very marked, but under average
conditions there seems to be little advantage over instruments of
moderate size for many classes of observations.
Certain it is that a great deal of valuable work is done with
comparatively small telescopes, ranging from six to fifteen inches
aperture, and this in all departments of astronomical research.
Hence, some of the most active observatories do not figure in the
above list; among them may be mentioned the observatories of Harvard
College, Potsdam, Paris, Heidelberg, Cape of Good Hope, Edinburgh,
South Kensington, Stonyhurst College, and the observatory of Dr.
Isaac Roberts at Crowborough, England.
HOUSING OF EQUATORIALS.—The building which accommodates an equatorial
telescope must evidently be designed to admit of giving a clear
opening to any part of the sky. Usually this is accomplished by
making the roof, or _dome_, with a circular base, provided with
wheels, which run on rails. It is then only necessary to open a
narrow portion of the dome, extending from top to base, and to turn
the dome until this aperture is in the required direction. One of
the most elaborate domes now in existence is that built by M. Eiffel
for the great refractor of the Nice Observatory. The lower part of
the building is in the form of a square, having a side of about
eighty-seven feet and a height of about thirty feet. The dome itself
is seventy-four feet in diameter, and the moving parts alone weigh
ninety-five tons.
There are two shutters, each a little wider than half the possible
opening; these run on short rails, and are moved simultaneously
by means of an endless rope. The whole of the dome is built up of
steel angle iron, covered with very thin sheet steel. In order to
facilitate the manipulation of the dome, its great weight is buoyed
up by means of a float attached to its base and immersed in a
circular tank of water of a little greater size than the base of the
dome. If any mishap occurs with this gigantic tank, the dome rests
on wheels which run on a circular rail, so that the work need not be
interrupted. The whole arrangement is very easily turned with the aid
of a winch by one man when the dome is floating, but when resting on
the wheels several men are required at the winch.
This brief description will serve to illustrate some of the problems
which confront the possessor of a very large telescope. For smaller
instruments, the observatories follow pretty nearly the same plan,
except that it is unnecessary to provide an arrangement for floating
the dome.
The observatory which shelters a reflecting telescope need not differ
very greatly from one which contains a refractor. If the instrument
be a Newtonian, it is generally convenient to sink the polar axis
below the level of the floor in order that the observer may not be
at too great a height from the ground, and in that case, the dome,
or its equivalent, is all that is necessary. For his five-foot
reflector, Dr. Common designed an observatory which is not of the
ordinary form, but gives the necessary opening partly by means of
large shutters and partly by a revolution of the whole house. It is
not every one who is able to lay out $40,000 on such a dome as that
erected at Nice by M. Bischoffeim.
The varying position of the eye end of a telescope, when it is
turned to different parts of the sky, makes it necessary to provide
comfortable and safe seating accommodation for the observer, more
especially when the telescope is a very large one. In the case of the
Yerkes telescope, the eye-piece is thirty feet higher when observing
near the horizon than when observing near the zenith, and the
observer must necessarily follow the telescope. The most convenient
arrangement in such a case is to raise or lower the floor of the
observatory as occasion demands. The floor of the Yerkes Observatory
is seventy-five feet in diameter, and by means of electric motors
it can be given a vertical motion of twenty-two feet. A similar
arrangement was provided for the Lick telescope from the designs of
Sir Howard Grubb. With smaller instruments, observing ladders and
adjustable chairs of various forms are employed.
THE EQUATORIAL COUDÉ.—A form of equatorial telescope which has
possibly a great future before it is one introduced at Paris under
the name of the _equatorial coudé_, or elbowed telescope. Its
practical advantage is that the observer remains in a constant and
comfortable position, so that revolving domes and elevating floors,
or other arrangements serving similar purposes, are no longer
necessary. The telescope tube is of two parts of nearly equal length,
and what is ordinarily the lower half of the tube forms part of the
polar axis, while the other half is attached to it at right angles.
At the point of intersection of the two halves of the tube is a plane
mirror, and there is another mirror in front of the object-glass. If
the latter mirror were removed, such a telescope would only enable
the observer to see objects lying along the celestial equator, but
by its means objects in all parts of the heavens can be brought
within range to an observer gazing down the hollow polar axis. The
largest instrument is that at the Paris Observatory, which has an
object-glass 23½ inches in diameter for visual observations, and
another of the same size for photographic purposes.
FIXED TELESCOPES.—There is still another method of using a telescope.
The telescope itself may be fixed, and the light of the heavenly
bodies may be reflected into it by means of a mirror which is made to
revolve so as to keep pace with their movements. Foucault devised an
instrument called the _siderostat_ for this purpose, and although
it is not largely employed for telescopic observations, it is very
widely utilized for spectroscopic work, where the spectroscope is of
a kind not readily attached to a telescope.
Another instrument used for the same purpose has recently been
brought forward under the name of the _cœlostat_. This is simply a
mirror which is made to turn on a polar axis in its own plane, and
since a reflected ray of light moves through twice the angle that
the reflecting surface turns through, the mirror is made to revolve
at the rate of one revolution in two days. As the name indicates,
the whole heavens appear stationary in such an instrument, whereas
in a siderostat only one star at a time appears at rest, while its
neighbors slowly revolve round it.
PHOTOGRAPHIC TELESCOPES.—The application of photography to the study
of the heavenly bodies marks one of the greatest advances of the
present century. The instruments which are employed for this purpose
range from the ordinary tourist camera to the largest telescope.
Unlike a person sitting for a portrait, the heavenly bodies can
not be made to stand still for the purpose, and as instantaneous
photographs can only be obtained in the case of the sun and moon, it
is usually necessary to make the camera follow the stars very exactly
during the time of exposure in order that the images may fall on
precisely the same parts of the photographic plate.
Some guiding arrangement is, therefore, essential, and generally
the photographic camera or telescope is attached to an ordinary
equatorial which is driven by clockwork, or very carefully by hand
if the camera be a small one. In the guiding telescope are two
spider-threads at right angles to each other, and it is by constantly
keeping the image of a star at the intersection of these “wires” that
the operator ensures the images remaining in a constant position upon
the sensitive plate.
An ordinary portrait camera, in the hands of a skilled observer,
yields very beautiful pictures, but they are naturally on a small
scale. The field of view of such an instrument is so large that a
whole constellation may be photographed with a single exposure.
Portrait lenses of six inches aperture in the hands of Dr. Max Wolf
and Professor Barnard have given magnificent delineations of the
Milky Way, and of the extremely faint nebulosities which are to be
found in many parts of the heavens.
For many purposes, however, telescopes of greater power are required,
and here it may be remarked that the distance between the images of
any two adjacent stars will vary in direct proportion to the focal
length of the telescope. In the same way the size of the image of a
planet, the moon, or a comet, increases as the focal length of the
objective is increased.
Refracting telescopes which are employed for photography require
object-glasses which are specially “corrected” for the photographic
rays. White light is compounded of light of all colors, but it is
the blue and violet constituents which are effective in producing
photographic action on an ordinary sensitive plate. Now, an
object-glass which is intended for visual purposes is made to
focus at the same point as many as possible of the rays which
are most effective to the human eye, that is the green, yellow,
and red, and usually there is a blue or purple halo round the
images of the brighter objects, which is, however, too feeble as
a rule to interfere with visual observations. This blue halo will
evidently result in defective definition if the lens be employed for
photography. By putting the plate at the point where the blue rays
are most nearly focused, a better image is obtained; but for really
good work a photographic object-glass must be so designed that all
the blue and violet rays are brought to one and the same focus. Such
a lens will consequently be a very poor one for visual observations.
The new “photo telescopic” object-glass now manufactured by Messrs.
Cooke appears to be full of promise. In this lens all the colors of
the spectrum are brought to almost exactly the same focal point, so
that it serves equally well for photographic or visual purposes.
This difficulty in regard to achromatism does not exist in the case
of the reflecting telescope, since rays of light of every color are
reflected at precisely the same angles. For this reason reflectors,
when properly managed, give the best photographic results. Dr. Isaac
Roberts and Dr. Common are especially identified with the application
of the reflecting telescope for celestial photography. The instrument
employed by the former consists of a 20-inch reflector and a 7-inch
guiding telescope of the refracting form. The two telescopes are
mounted on the extreme ends of the declination axis of an equatorial.
Dr. Common does not employ a guiding telescope at all. The
photographic plate which he places at the focus of the reflector is
smaller than the field of view, so that by means of an eye-piece
fitted with a cross wire at the side of the dark slide, he is able
to watch a star near the edge of the field. Both eye-piece and dark
slide are attached to a frame which can be controlled by two screws
at right angles to each other. If the guiding star leaves the cross
wire through errors in driving, or other causes, the eye-piece and
dark slide are bodily moved after it by means of the adjusting
screws. This method not only has the advantage of saving the cost of
a guiding telescope, but reduces the effects of vibration consequent
upon the correction of errors by moving the whole telescope.
For photographing the sun a special instrument called a
_photoheliograph_ is usually employed. This differs only from an
ordinary photographic telescope in being provided with a secondary
magnifier, by which means the focal image formed by the object-glass
is amplified before falling upon the photographic plate. On a bright,
clear day pictures of the sun eight inches in diameter can be taken
with an exposure of about 1/500th of a second, and such a photograph
will frequently record more facts as to the state of the solar
surface than a whole day’s observation. Lenses or mirrors of very
long focus are also occasionally employed in solar photography, and
in this way a large image is obtained without the use of a secondary
magnifier.
Photographs of the moon and planets may be taken either with or
without a secondary magnifier, but in either case the exposures are
longer than for the sun.
Finally, it may be added that the sensitive plates and processes used
in astronomical photography do not differ from those employed by
ordinary photographers.
FOOTNOTES:
[22] The focal length of a lens is the distance from its centre at
which an image of a very distant object, such as the sun, is formed.
METEORS.—SIR ROBERT S. BALL
Our present knowledge as to the natural history of the shooting stars
has been mainly acquired during the last hundred years. The first
important step in the comprehension of these bodies was to recognize
that the brilliant flash of light was caused by some object which
came from without and plunged into our air. This was known at the end
of the Eighteenth Century, largely by the labors of the philosopher
Chladni in 1794.
[Illustration: A Portion of the Moon’s Disk
Where Four Mountain Ranges Meet]
Could an ordinary shooting star tell us its actual history, the
narrative would run somewhat as follows: