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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.