The astronomer will not be contented with a mere naked-eye inspection
of a world so interesting as the moon. He will get a telescope to help
his vision. The word “telescope” means a contrivance for looking at
objects which are a long way off. We have explained that the further
an object is, the smaller it appears to be. The telescope enables us
to largely overcome this inconvenience. It has the effect of making a
distant object look larger.
There are great differences in the forms of telescopes; and some
instruments are large and some small, according to the purposes for
which they are required. Perhaps the most useful practical application
of the telescope is by the officer on duty on board a ship. He is
generally provided with a pair of these instruments bound together to
form the “binocular.”
You are all acquainted with this useful contrivance, or at all
events with the opera-glass, that is used for purposes with which
landsmen are more familiar. The ship’s telescope, or the binocular,
or the opera-glass, is feeble in power when compared with the great
instruments of the Observatory. The officer on the ship will generally
be satisfied with a telescope which shall show the objects with which
he is concerned at about one-third of their actual distance. Thus,
suppose his attention is directed to a great steamer three miles
away, he wishes to see her more clearly, and accordingly he takes a
view through his binocular. Immediately the vessel is so transformed
that it seems to be only one mile away. The apparent dimensions of
the object are increased threefold. The hull is three times as long,
the masts and the funnel are three times as high, the sailors are
three times as tall; various objects on the ship too small to be seen
at three miles would be visible from one mile, and to that apparent
distance the ship has now been brought.
If the sailor desires to reduce the apparent distance of objects, how
much more keenly does the astronomer feel the same want? At best, the
sailor only has to scan a range of a few miles with his glass, but what
are a few miles to the astronomer? It is true that he can count the
distance of the moon by thousands of miles, a good many thousands, no
doubt, but for all other objects he must use millions, while for most
bodies in space, millions of millions of miles are the figures we are
constrained to employ. Need it be said that the astronomer must resort
to every device he can to make the body appear closer. He does not
despise the modest binocular. It is often a useful instrument in the
Observatory. It gives most beautiful pictures of the celestial scenery,
and you would be amazed to find how many thousands of stars you can
see with its help which your unaided eye would not show you at all.
The binocular will also greatly improve the appearance of the moon,
but still its powers fall far short of what we require for the study
of lunar landscapes. Even though we can reduce the moon’s apparent
distance to one-third its actual amount, yet still that third is a
very considerable distance. One-third of 240,000 is 80,000, so that we
can see the moon no better with a binocular than we should see it were
it 80,000 miles away, and were we viewing it with the unaided eye.
[Illustration: FIG. 36.--The Dome at Dunsink Observatory.]
[Illustration: FIG. 37.--The Equatorial at Dunsink Observatory.]
I am not going to enter here upon any detailed account of the
telescope, because I shall say a little more on the subject in a later
lecture; at present I only describe that form of instrument which is
most convenient for studying the moon. I take as an illustration the
South Equatorial at Dunsink Observatory, which belongs to Trinity
College, Dublin.
This telescope has a building to itself, which stands on the lawn in
front of the house. The site is open and elevated, so as to command an
extensive prospect of the heavens. You will see in Fig. 36 a picture
of the structure. It is circular in form and is entered by the little
porch. The most peculiar feature of an edifice intended to contain
this kind of telescope is its roof, or _Dome_, as we call it. It is of
a hemispherical shape with a projecting rim at the bottom. But no one
would go to the trouble and expense of making a round dome like that
over the Observatory if it were not necessary for a particular purpose.
The dome is very unlike ordinary roofs, not only in appearance, but
also because it can turn round. In the next figure you will see a
section through the building, and the wheels are exposed by which the
dome is carried. These wheels run easily on rails, so that when the
attendant pulls the rope which you see in his hands, he turns round a
large pulley, and that operates a little cogwheel which works into a
rack, and thus makes the dome revolve. The roof is built of timber,
covered with copper; it weighs more than six tons, but the machinery
is so nicely adjusted, that a child four years old can easily set
the whole in motion. The object of all this machinery is seen when
we learn that there is only one opening in the dome. It is covered
by the shutter shown over the doorway in Fig. 36. When opened to the
top, it gives a long and wide aperture, through which the astronomer
can look out at the heavens. Of course the dome has to be turned until
the opening has been brought to face the required aspect. The big
telescope can thus be directed to any object above the horizon. You
see a gentleman using the telescope (Fig. 37), and this shows that
the great instrument is nearly three times as long as the astronomer
himself! No doubt the telescope seems to be composed of a good many
different parts, but the essential portions of the instrument are
comparatively few and simple. At the upper end is the object glass,
which consists of two lenses, one of flint glass and the other of crown
glass. Both of these must be of exceptional purity, and the shape to
be given to the lenses is a matter of the utmost importance. It is in
the making of this pair of glasses that the skill of the optician has
to be specially put forth. So valuable indeed is an object glass which
fulfils all the requirements, that it is by far the most costly part
of the instrument. There are no glasses in the interior of the tube
until you come to the end where the observer is looking in. This is
closed by an eyepiece consisting of a lens, or a pair of lenses. There
are usually many different eyepieces for a telescope, and they contain
lenses of varied powers, to be used according to the state of the
atmosphere, or to the particular kinds of observation in progress.
If you point a big telescope to the sky, and see therein the sun or
the moon or any of the stars, you will speedily find that the objects
pass away out of view. Remember our earth is constantly turning
round, and bears, of course, the Observatory with it, so that though
the telescope be rightly pointed to the heavens at one moment, by
the next it will have been turned aside. To you who are using the
telescope, the appearance produced is as if the heavenly bodies were
themselves moving. We can counteract this inconvenience. The telescope
is supported on a pedestal, which is built on masonry, that goes down
through the floor to its foundation on the solid rock beneath. In the
iron casing at the top of the pedestal you will see a little window,
and inside is clockwork driven by a heavy weight. This clockwork turns
the whole telescope round in the opposite direction to that in which
the earth is moving. The consequence is that the telescope remains
constantly pointed to the same part of the heavens.
[Illustration: FIG. 38.--The Yerkes Telescope, University of Chicago.]
This instrument is no doubt a large one, but of late years many much
greater have been built. The most powerful telescope that has ever been
erected is the great Yerkes instrument belonging to the University of
Chicago, of which a picture is shown in Fig. 38. The object glass is 40
inches across.