THE SPYGLASS OR TELESCOPE
=The Boy Who Discovered the Telescope.=—Spectacles have been made and
used for nearly a thousand years and the art of making lenses is very
much older.
A little over three hundred years ago there lived in Amsterdam,
Holland, a spectacle maker named Lipperhey, and it is said of him that
he made good lenses.
There was apprenticed to this lens grinder a Dutch boy, and I am sorry
I cannot tell you his name, for he was a boy who did things; but his
name is not recorded, which is a shame, for if it had not been for this
boy Galileo might never have had a telescope.
One day while the boss was out this Dutch boy was standing before a
window of the shop and he held a lens before his eye with one hand and
another lens before the first lens with his other hand, as shown in
Fig. 148. Imagine his surprise when the church he was looking at seemed
to move much nearer to him, that is to say, the image of the church was
greatly enlarged.
The boy had made a wonderful discovery—he had discovered the
_telescope_. When his master returned the boy showed him what he had
done and it was not long before the great Galileo had a telescope and
was startling the world by his wonderful discoveries of the moons of
Jupiter, the rings of Saturn, the phases of Venus, the spots on the Sun
and a hundred other wonders of the sky.
A telescope is an arrangement of lenses in a tube for making the image
of a distant object larger on the retina of the eye, or, as in the case
of the fixed stars, for making them brighter.
[Illustration: FIG. 148.—LIPPERHEY’S BOY DISCOVERS THE TELESCOPE.]
The word telescope comes from two Greek words, the first, _tele_,
which means afar, and the second, _scope_, which means to see, so that
telescope means just what we should expect it to mean and that is to
see afar.
There are several kinds of telescopes, but there are only two kinds
I want to make clear to you here; in the first kind the eye sees the
object just as it is, that is, standing right side up, or _erect_. This
kind of a telescope is called a _spyglass_, and is used to look at
objects on the surface of the Earth.
In the second kind of telescope the eye sees the object upside down,
or _inverted_, and this kind of telescope, which is called an
_astronomical telescope_, is just as good as the other for looking at
the stars.
=A Pinhole Telescope.=—Before describing how to make and use real
telescopes which have lenses I want to tell you of a little scheme to
see afar, and though it does not magnify the image of the object that
is seen through it, yet it aids the naked eye when you are looking at
the Sun and Stars.
To make a pinhole telescope get a pasteboard tube about 1¼ inches in
diameter and 5 or 6 inches long. A paper tube for mailing papers and
sheet music is just the thing and can be bought at any stationery store.
[Illustration: FIG. 149.—DISK OF CARDBOARD FOR PINHOLE TELESCOPE.]
[Illustration: FIG. 150.—CROSS SECTION OF PINHOLE TELESCOPE.]
Cut out a _disk_, or circular piece of cardboard just large enough to
fit the tube, see Fig. 149, and push a pin point or needle through the
center to make a small, clean hole. Next, glue this cardboard disk in
the tube ½ an inch from one end. The disk must be glued in the tube so
that no light can leak around the edge.
If, now, you look at the Sun through the pinhole telescope, as shown
in Fig. 150, you will get a better view of it than if you look at it
through a pinhole in the cardboard alone, for the tube shuts out all
the other rays of light from the eye.
To improve the seeing qualities of the pinhole telescope make the hole
in the cardboard disk ¼ inch in diameter and cover this hole with a bit
of tinfoil. Now make a hole in the center of the tinfoil with the point
of a needle; this makes the edge of the hole sharp.
A pasteboard tube without the pinhole will also aid the naked eye in
seeing the Moon and stars, for it shuts out all the rays of light
around the eye and limits the sight to a certain part of the sky;
together these things are very good helps in observing especially if
there are gas and electric lights nearby.
[Illustration: FIG. 151.—THE TELESCOPE (GALILEO).]
=How a Telescope Works.=—A real telescope has at least two lenses in
it; the larger lens is placed in the end of the tube nearest the object
to be viewed and is called the _object glass_ because the light from
the object is received by it.
The smaller lens is placed in the end of the tube nearest the eye,
and is called the _eyepiece_, for it is this lens which enlarges, or
magnifies the image of the object that is thrown upon the retina of the
eye.
There are two simple kinds of telescopes; the first is the kind that
Galileo used for making his great discoveries. The kind of lenses used
and the way they are placed in the tube is shown in Fig. 151.
In this telescope the object glass is a double convex lens and the beam
of light which strikes it is brought to a point as in the case of a
burning glass, but before the light reaches this point it is caught up
by the double concave lens which forms the eyepiece, when it is carried
to the eye in an erect position.
[Illustration: FIG. 152.—OPERA GLASSES.]
An opera glass is simply a pair of these little telescopes, joined
together so that they can be focused at the same time by means of an
adjusting screw, as shown in Fig. 152.
Another simple telescope is formed of two double convex lenses. As
in the telescope just described the larger lens, or object glass, is
placed in the end of the tube nearest the object to be viewed and the
smaller lens, or eyepiece, is placed in the tube nearest the eye.
In this telescope, though, the eyepiece is a double convex lens and
while a larger image is formed in the eye it is inverted, or upside
down, so that this kind of a telescope is of no use as a glass to spy
things with on the surface of the Earth.
=How to Make a Cheap Telescope.=—Lenses are absurdly cheap. If you live
in a large city you will find lens grinders who will sell you the kind
of lenses you need for either kind of telescope for a dollar or so.
=Telescope No. 1.=—This telescope is fashioned after an opera glass,
that is, it has a concave lens for an eyepiece.
[Illustration: FIG. 153.—PASTEBOARD MOUNTING OF LENS.]
[Illustration: FIG. 154.—PASTEBOARD LENS MOUNTING.]
Get two pasteboard tubes of the kind described for the pinhole
telescope; the bore, or hole, of the first tube should be 1¾ inches in
diameter and it should be 2 inches long. Have the second tube a little
smaller than 1¾ inches in diameter on the outside so that it will slide
easily into the larger tube and yet not leak light, and have this tube
1½ inches long. Paint the inside of both tubes with black paint to keep
the walls of the tube from reflecting any stray rays of light which may
strike them. The outside of both tubes can be covered with bookbinders’
cloth to give them a neat appearance.
The next step is to mount the lenses. The larger lens for the object
glass is a double convex lens 1½ inches in diameter and having a
4-inch focal length, or _focus_, as it is called for short. A lens of
this kind can be bought for 25 or 30 cents. The smaller lens for the
eyepiece is a double concave lens 1 inch in diameter and having a focus
of 2 inches. This lens can be had for about 40 cents.
[Illustration: FIG. 155.—OPERA GLASS TELESCOPE. CROSS SECTION.]
Cut a strip of thin, tough cardboard ½ inch wide and 6 inches long; on
this strip glue a strip of heavy pasteboard ½ inch wide and 5⅜ inches
long, and have one end of both pieces even, as shown in Fig. 153. Set
a flatiron on the pieces and let them dry. When dry make a groove down
the middle of the heavy pasteboard by slicing out a very thin strip
with the point of a sharp knife, being careful not to cut through the
thin cardboard. Now bend the strips around the lens with the lens in
the groove, glue over the thin end as shown in Fig. 154, and slip a a
rubber band or tie a string around it to hold it in position until it
has dried.
The small concave lens, or eyepiece, is mounted in the same way, but
since the lens is only 1 inch in diameter, cut the strip of cardboard ½
inch wide and 4¼ inches long and the strip of thick pasteboard ½ inch
wide and 3½ inches long; this done glue them together, cut the groove
and mount the lens as before.
[Illustration: FIG. 156.—TELESCOPE. CROSS SECTION VIEW.]
The next and last thing is to smear glue on the cardboard mounts and
push the convex lens in the end of the large tube and the concave lens
in the end of the small tube. Now slide the tubes together and you will
have as good a telescope as the boy who invented it. It is shown in
cross section in Fig. 155.
You should, however, make two caps, one for each end of the telescope
to cover the lenses when not in use. This little telescope is very
handy to carry along on your scouting trips, as it takes up so little
room, being only 2 inches long when closed up.
=Telescope No. 2.=—This telescope is very much better than the one just
described for seeing the stars as it magnifies about 4 times.
It is made exactly like the first one except that the larger tube is 14
inches long and the smaller tube is 5 inches long. In this telescope
both lenses are double convex, the large one, or object glass, having
a diameter of 1½ inches and a focal length of 12 inches, while the
smaller lens, or eyepiece, has a diameter of 1 inch and a focal length
of 3 inches. It is shown in cross section in Fig. 156. A spyglass
usually has four or five _plano-convex lenses_ in it and these not only
magnify the image but they also erect it so that you see the object as
it really is.
While the homemade telescopes which I have described will not magnify
as highly as a cheap telescope which you can buy, yet you ought to make
one, for it will let you into the secret of combining lenses, and this
is as interesting as seeing the stars. By all means make your first
telescope and then if you want a better one buy it and get one as large
as you can afford.
[Illustration: FIG. 157.—MAGNIFYING POWER OF TELESCOPE.]
=To Find the Power of a Telescope.=—In the last chapter, I explained
what the _focal length_ of a convex lens is, see Fig. 146, and how to
measure it. To repeat, it is the distance in inches between the center
of a lens and the point where the rays come together.
You can find exactly what the _magnifying power_ of your telescope is,
when both lenses are convex, by dividing the focal length of the object
glass by the focal length of the eyepiece, or lens.
For instance, suppose the focal length of the convex object glass of
your telescope is 12 inches and the focal length of the convex eye lens
is 3 inches, then 12 ÷ 3 = 4 and the quotient 4 is the magnifying power
of your telescope.
To find the focal length of a concave lens is a little harder, but
Garrett P. Serviss tells us in his good little book on _Astronomy with
an Opera Glass_ of an easy way to judge the magnifying power of an
opera glass and it is just the same for a telescope. Look at a brick
wall through one of the tubes with one eye while the other naked eye
sees the wall direct.
Now notice how many bricks which the naked eye can see, are needed to
equal the thickness of one brick as seen through the glass. The number
of bricks seen with the naked eye represents the magnifying power of
the glass. Fig. 157 shows how the bricks are compared.
=The Stars Seen Through a Spyglass.—The Moon.=—When Galileo lived the
people believed that the Moon was a ball as smooth and bright as a
glass marble and they also thought that the dark spots on its surface
were the continents of our Earth reflected by it.
So the first thing Galileo did when he got his telescope was to turn it
on the Moon, for he wanted to know about these dark spots, and you can
imagine his surprise and delight to find that they were really great
mountains and extinct volcanoes.
You cannot do better than to point your little homemade telescope at
the Moon, stop, look and rediscover the mountains on it and be as
surprised and delighted as Galileo was, three hundred years ago. To see
the mountains at their best do not wait until the Moon is full, for the
sunlight then shines directly on top of the mountains and there are no
shadows to help the eye to gauge breadths and heights.
The best time to see the mountains is when the Moon is in its first
or its last quarter, for then they are well brought out by the bright
sunlight shining on them from the side and the black shadows which they
cast on the other side.
The great smooth stretches seen on the Moon are called seas. It may be
that in the long ago they were really seas, but it is more likely that
Galileo and the early observers whose telescopes were little better
than yours thought they were seas. Then there are huge cracks or gorges
on the surface of the Moon, which start from some of the craters and
run for hundreds of miles in every direction. A number of these gorges
start from a volcano named _Tycho_ (pronounced Ti´-co) and make the
Moon look as if it is cracked; and it is likely that when the Moon
cooled down from its melted state after having been shot off from the
Earth it did crack in many places. Fig. 158, which is a good telescopic
view of the Moon, shows some of these great cracks radiating from Tycho.
[Illustration: FIG. 158.—FULL VIEW OF MOON.]
To show on a small scale how the Moon cracked on cooling down Nysmith
filled a glass globe with cold water and then sealing the globe he
plunged it into hot water. The slow expansion of the cold water by
the hot water caused the globe to crack as shown in Fig. 159, and by
comparing the pictures it will be seen that the cracks on the Moon and
in the glass globe are very much alike.
There is a mountain called Aristarchus[1] (pronounced Ar-is-tar´-cus)
which is believed to be formed of pure metal because it shines brighter
than any other mountain on the Moon. Its position is shown on the map,
Fig. 160.
[Illustration: FIG. 159.—GLASS GLOBE CRACKED.]
The instant you look through your glass at the Moon the man which
shows so plainly to the naked eye vanishes like a coin in a magician’s
fingers and instead you will see a new world covered with plains and
mountains.
But if you will look at the Moon when it is nine or more days old, you
will see with the aid of your glass and a little imagination the Moon
girl, as shown in Fig. 161. You should have no trouble in finding her,
for the _Apennines_ form her crown while Tycho shines upon her breast
like a great yellow diamond.
[Footnote 1: Aristarchus was a Greek who lived 200 years before Christ.
He taught that the earth was round.]
[Illustration: FIG. 160.—MAP OF THE MOON.]
[Illustration: FIG. 161.—THE MOON GIRL.]
There is another crater mountain which you should by all means know
and that is Copernicus[2] (pronounced Ko-per´-ni-kus). Look through
your glass at the southeastern end of the Apennines and you will see a
crater that is larger in diameter than Tycho, though it is not so deep.
Tycho, Copernicus and the Apennines will serve you well as landmarks if
you follow up the explorations of the Moon which you have so well begun.
[Footnote 2: Copernicus—Polish, astronomer. Born 1473. Before his time
it was believed that the Sun, Moon and stars revolved round the Earth.
Copernicus showed that the Earth was a planet and, with the others,
that it revolved round the Sun.]
=The Sun.=—You can see the spots on the Sun quite well with your glass,
and sunspots are always interesting.
Do not try to look at the Sun through your glass without covering the
eyepiece either with a thickly smoked or dark-colored glass. You must
also use smoked or colored glasses over your eyepiece when you are
looking at an eclipse of the Sun.
You should look for sunspots this year, 1920, for they are decreasing
in number and size and in a few years there will be scarcely any. After
that there will be more of them again.
=The Planets.=—After you have seen the mountains on the Moon and the
spots on the Sun you should next turn your glass on the planets.
While you will probably say that the planets loom up very small—the
largest, Jupiter, appearing about the size of the head of a large
pin—yet they are wonderful to look at even through the smallest
telescope.
=Mercury.=—This is a planet not easy to see even with the aid of a
glass so if you see it you can think that you are lucky or skillful or
perhaps a little of both.
=Venus.=—Venus can be seen very much better with your small glass than
Mercury, but you will only be able to see it as a little disk and not
as a crescent for your glass is of too low a magnifying power. It is a
brilliant object though even through the smallest glass.
=Mars.=—While you cannot see the canals of Mars with your glass you can
see it as a bright disk of light and this is well worth while. Mars is
believed to be peopled and the thought that it may be makes it a mighty
interesting object to look at. Look at it through your glass and think
it over.
=Jupiter.=—On account of his great size you will be able to see Jupiter
better than any of the other planets. His disk will show clear and
distinct and if you have good eyesight and your glass is fairly good
you will be able to see one and perhaps two of his nine moons which
will appear as little points of light close to the planet.
=Saturn.=—The rings of Saturn cannot be seen with a glass magnifying
less than four times. His rings are at this writing (1915) in the best
position to be seen as the flat side of the rings is toward us now. In
1921 the edge of his rings will be in a line with the Earth and then it
will be very hard to see them even with a much larger telescope.
=Uranus.=—Although Uranus is so very far away, it can be seen with your
glass, though you may not be able to see it as a disk of light.
You can tell when you have found Uranus by watching it for a few
nights. If it changes its position among the stars around it you will
know you are looking at Uranus.
=Neptune.=—Neptune is farther away than Uranus and your glass will show
it as a mere point of light. Like Uranus you will know it if you see
it, by its motion among the stars.
=The Stars.=—After you have looked at the Sun, Moon, and planets to
your hearts’ content turn your glass on the Big Dipper and you will see
about ten times as many stars as you can see with the naked eye.
_The Big Dipper_ is full of starry surprises as you will find to your
pleasure on looking at it with your glass: for instance, instead of the
handle being formed of three stars, it blazes with dozens of them.
Take a look at Alcor, which is the middle star in the handle of the
Dipper. You may remember I told you in the first chapter that it had
a little companion, Mizar, which only sharp eyes can see. Now look at
Alcor through your glass and you will see that it and Mizar are quite
widely separated.
The North Star is also a double star as these twin stars are called.
Just as Mizar is a good test for the naked eye so the twin of the North
Star is a good object on which you can try out the seeing power of your
glass.
Another double star which some boys can separate with their keen naked
eyes is _Epsilon_, named after one of the Greek letters (See Appendix
C). This star together with Vega, a very bright and beautiful blue star
of the first magnitude, and Zeta, another Greek letter star, form a
_triangle_, which is the constellation of Lyra.
Whether or not you can separate Epsilon into two stars with the naked
eye, you will see them stand out separate and distinct through your
glass. If you had a more powerful glass you would see that each of the
stars of Epsilon has a faint companion star, so that it is really a
_quadruple star_, that is, there are four stars right together.
Then there is the Milky Way, always a wonderful sight to the naked eye,
but still more wonderful when viewed through a glass however small;
there are the stars of the Pleiades of which the eye sees not more than
six or seven without help, but which bursts forth like a skyrocket into
a cluster of many-colored lights when seen through a glass; these are
only a few of the hundreds of other things which you can see in the sky
with the help of your little telescope.