Photographing the Stars.—There are some interesting things you can do with an ordinary camera in the way of photographing the stars.

A camera is made up of three principal parts, and these are (1) a convex lens, which forms the image of the object to be photographed; (2) a light-tight box, which keeps out all the light except that which passes through the lens, and (3) a plate, or film holder, which holds the sensitive plate, or film, in the camera and keeps it perfectly dark until you are ready to have the image formed on it.

The instant the lens is allowed to form an image on the sensitive plate, or film, we get out of the optics of photography and into the chemistry of it.

The sensitive plate, or film, is coated with a thin layer of salts of silver and bromide, heated with gelatine and a little water, and when these substances are thoroughly mixed, an emulsion results, and this is spread on glass plates or on celluloid films. When the plate is exposed, that is, when the image has been formed on it, it is developed. The developing is done by placing the plate in a solution of pyrogallic acid, or hydroquinone and water.

The plate, or film, is now put in the fixing bath, which is simply a solution of hyposulphite of soda and water. This fixing bath prevents any further action of the light on the plate, or film.

But the picture on the glass plate, or celluloid film, looks all same like a Chinese chromo, for the light and dark parts are just the reverse of that of the object which was photographed; in other words, where the picture should be white, it is black, and where it should be black, it is white, and this is the reason it is called a negative.

To get a picture of the object, as it looks to the eye, a sheet of paper, also coated with salts of silver, is laid flat on the negative and held close to it in a printing frame when it is exposed to the light of the Sun.

When the paper is printed dark enough by the Sun, it is toned to give it a pleasing color and fixed to make it last a long time, and when washed and dried the image of the object, true to life, is there, a wonderful record for all time, nearly.

This then, briefly, is how the Sun, or other source of light, makes an image, how the image is fixed on a dry plate, or film, and how a picture is printed from the negative, and having found out this much let’s see how we can make it do a little star work for us.

There is not much we can do in the way of making pictures of the things in the sky with a small camera, but the few things we can do are mighty interesting, and show the stars in a way you can never see them with the naked eye.

To make star trails is one of these interesting things. First, set your camera on a tripod, or other firm support, and focus it on a tree or something as far off as you can see it in the daytime. Now, on a dark night, when there is no Moon, set your camera so that the lens is pointing directly at the North Star, as shown in Fig. 180. Set the shutter of your camera for a time exposure and open it.

You can go to bed now and let the stars work for you while you sleep, that is if you can get up before daylight the next morning, but if you cannot do this stay up as long as you can, three or four hours, anyway, before you close the shutter.

Now, when you develop the plate, or film, and make a print from it you will have a record of the apparent path of the stars, as shown in Fig. 181, but which is, of course, due to the Earth turning round on its axis. If you set your camera with the lens pointing toward the ecliptic, that is, the path of the Sun, the star trails will be long, straight lines, and this will make another interesting record.

By pointing your camera toward that part of the sky and during that time of the year when there are showers of meteors, and opening the shutter of your lens, you stand a chance, though it is not a very fat one, of catching one of these wily shooting stars on your plate, and if you do succeed—well, you will have a picture that is a curiosity.

You can photograph the Sun with an ordinary camera and make his disk as large as you want to, but you will get nothing more on your plate than a white spot. The Moon can likewise be photographed, but if you focus it sharp on the plate it will not be much larger than a mere point of light, and if you get a disk large enough to see there will only be a small white spot on your finished print.

To make good photographs of the Sun, Moon and planets, a big telescope driven by clockwork to offset the turning of the Earth is needed. Such a telescope is called an equatorial telescope, and when it is set so that an object in the sky is in the field of view, it will remain right there as long as you want it, and when you make a photograph of the object it will be as sharp and as clear as though both it and the Earth were standing perfectly still.

If you think enough of the stars to try to photograph them with your little camera, I should say there is a very good chance of your being able to photograph them sometime through an equatorial telescope for all things come to the boy who wants them hard enough.

What the Stars Are Made of.—To know what the stars are made of is to know more about them than men knew of the Earth a few hundred years ago.

But just think of looking at a star like Aldebaran, which is so far away that it takes 45 years for its light to reach the Earth—light travels eleven million miles a minute—and then saying it is made of iron, and mercury and hydrogen and sodium and half a dozen other substances!

Now you ask, “How is it done?” And I’ll say with a glass prism, and then we’ll have made a flying start. Now a glass prism is a three-sided piece of glass—the ends don’t count—as shown in Fig. 144, and it is just as wonderful in its way as a lens is wonderful in its way.

If you will hold a prism to your eye and look at the flame of a candle through it, you will see the flame in all the brilliant colors of the rainbow, and these bright colors form what is called a spectrum.

Now make another experiment; cover a window, on which the Sun shines, with a sheet of cardboard, in the middle of which you have cut a horizontal slit with a sharp knife, about 1 inch long and ¹/₂₅ inch wide. Make the room perfectly dark except for the light which comes in through the slit in the cardboard, and set a prism in front of the slit, as shown in Fig. 145. The beam of sunlight which passes through the prism will be split up, or decomposed, as it is called, into the seven colors of the rainbow, but the colors will be much brighter.

If you will fix another sheet of white cardboard in a vertical position so that the colors will shine on it in a band, the colors, beginning with red at the bottom, next orange, then yellow, green, blue, indigo and violet, will follow each other to the top bright and beautiful. These rainbow colors, spread out in a band on the screen, form what is called the solar spectrum, that is, the spectrum which is produced by sunlight.

This beautifully colored spectrum is really made up of a great many images of the slit in the cardboard, one for each wave-length of the light. If the slit is made very narrow, the images of the slit will not overlap much and you can tell whether any of the wave-lengths are missing by looking for fine black lines crossing the spectrum. Such dark lines are very important in finding out what the Sun and Stars are made of.

All you have to do is to stand away from the slit five or six feet and look at the beam of light through a prism just as you did at the flame of the candle. This is the simplest form of a spectroscope, that is an instrument for splitting up the light of any object which is self-luminous, into a spectrum, as shown in Fig. 182.

When you look at the sunlight streaming in through the slit in the cardboard you will see a number of dark lines crossing the colored band. These dark lines are called Fraunhofer’s lines, because Fraunhofer, who was a German telescope maker, was the first to show how important they are. The chief dark lines of the Sun’s spectrum are known by the letters of the alphabet, and, like the colored images of the slit, they are always in the same place. They are shown in Fig. 183.

Having found out a little about the spectrum and how it is formed by the Sun, let’s find out next how it tells us what the Sun and the stars are made of.

Now, in the Sun’s spectrum there are a pair of lines close together and near the middle which are called the D lines—see Fig. 183—and when a beam of sunlight is split up by a spectroscope, or rather by the prism of a spectroscope, these D lines always appear in the yellow part of the spectrum.

There was a time, and not so very long ago, when no one had the faintest idea why the Sun’s light did not contain these two wave-lengths, when the spectrum from a candle flame was a continuous band of light without dark lines. Then two experimenters, Kirchhoff and Bunsen, found that various substances when converted into luminous gases in a hot flame always produced spectra which consisted only of certain bright lines. Thus it was found that sodium gas gives two bright lines in the exact positions of the D lines of the Sun’s spectrum. This made it look as if the Sun contained no sodium. But the experimenters did something else: they placed a very brilliant light back of the sodium gas and on looking through their spectroscope they saw a continuous spectrum with black D lines. This meant that the sodium gas had absorbed from the light passing through it the wave-lengths which it had given out when shining by itself. We now know that the Sun is surrounded by a layer of gases which is somewhat cooler than the interior of the Sun and that each substance in this “reversing layer,” as it is called, takes out some of the light which is characteristic of it and thus produces the Fraunhofer lines.

In the same way iron and other metals and sodium and other substances which are heated until they become gases, and hydrogen and other gases which are aflame, produce spectra consisting of certain bright lines, and as the light from the Sun also produces black lines in the same identical places, it is known that the Sun contains these different substances which are heated up in it to a white heat.

It’s the same thing with the stars. It makes no difference if the light is made by burning some substance a few inches away from the slit of the spectroscope, or whether it has traveled 90 millions of miles from the Sun—which takes about 8½ minutes, or 45 years for the light to reach it from Aldebaran, it always acts exactly in the same way; and so we know a good deal about the stuff the stars are made of.

Let’s see now, how a real spectroscope is made, for it isn’t always convenient to have a pitch dark room, nor is it a very exact way to hold the prism to your eye when examining the light of burning sodium, or hydrogen gas, or other substances.

The spectroscope is an instrument made so that the light of a substance burned at one end will pass through a prism and will form a spectrum on the retina of the eye at the other end. It is usually made up of two tubes mounted on a stand at an angle with the prism between them, as shown in Fig. 184.

In the end of the first tube, which is called a collimator, a very narrow slit is cut, and it is at this end that the substance is placed which is to be burned and whose light is to be split up and examined.

A convex lens is fitted in the other end of the collimator, or first tube, so that the light after passing through the slit will then pass in a beam through the prism. The other tube—the one you look through—is called a telescope and in this one is fitted a convex object glass and an eyepiece which magnifies the spectrum made by the prism.

When a camera is used to photograph the spectrum the eyepiece is taken out of the tube of the spectroscope and a little camera is put in its place. In this way the camera and the spectroscope are combined and this helps chemists to compare the spectra of different burning substances far better than they could do with the naked eye alone.

To photograph the spectrum of the Sun the eyepiece of the big telescope is taken out and the end of the tube of the spectroscope with the slit in it is fitted in its place. In making a photograph of the spectrum of a star the slit is not needed, for the light of the star itself is but a mere point.

In photographing the spectrum of the Sun and stars three wonderful instruments are combined, namely, the telescope, the spectroscope and the camera; perhaps I should have said four wonderful instruments, for without the brain of a genius using them the Sun and stars never would have given up their secrets.