How the Stars Shine.—A burning match, a candle, an oil or gas flame, the Sun, comets and meteors all give out light and heat in exactly the same way, but the heat and light are made differently.

When you strike a match the friction makes enough heat to light the chemicals of which the head is formed and the burning gases light the splint which in turn generates more gases from the wood and these give out more light and heat.

When you light a candle the heat melts the wax which is then drawn up the wick, the burning gas around the wick produces more gas and the gas keeps the flame going. In the case of an oil lamp the oil, which is already a fluid, is drawn up by the wick, where it is changed into gases, and light and heat result as in the candle. The oil lamp is, then, a step ahead of the candle, for the solid wax is replaced by the fluid oil.

In the gas light the gas, which is made of coal or other matter, is forced out of the jet under pressure and this gives a bigger and better flame than the oil lamp; and, as the oil lamp is better than the candle, so the gas jet is an improvement over the oil lamp.

Now the Sun, and this is also true of most of the stars, is so large and hot that if any solid matter, such as iron and other metals could be thrown into the Sun they would not only be melted but instantly changed into gases. Further, the Sun is so hot that the materials cannot combine with Oxygen—in other words they cannot burn. The intensely hot gases of the Sun radiate the light and heat and we suppose that they keep themselves hot largely by contraction.

When a comet comes close enough to the Earth to be seen it is then close enough to the Sun so that the light and heat of the Sun cause some of the gases of which the nucleus of the comet is formed, to become white hot; as a comet gets closer to the Sun the solid matter of the nucleus, such as sodium—which is a kind of salt—iron and other things are changed into gases and these burn fiercely.

While we can see the light of a comet we cannot feel its heat, for a comet is too small to send its heat waves through such a great distance.

Just as a match is lit by striking it, so meteors are set on fire by striking the air. Meteors are made up of the same kind of matter as comets and when these shooting stars come within the attraction of the Earth the friction caused by the meteor rubbing against the air is so great that an intense heat is produced and the gases burst into flame.

If a meteor is small it is entirely burned up before it reaches the Earth and all we see of it is a bright streak of light. If a meteor is large enough only the outside of it is burned and it will, in consequence, reach the Earth, when it becomes, as explained in the last chapter, a meteorite.

Meteors and meteorites produce a very bright light, burning as they do in the oxygen of our air, and though they are very close to us they are so small we cannot feel any heat sent out by them.

We have said that when a match is struck the friction produces heat and that when the wax of a candle, the oil of a lamp or the gas of a jet is burned they produce light and heat, and this is also true of the blazing Sun, the fiery comets and the burning meteors.

Where there is light there is usually heat and turn about where there is heat there is light if the temperature is high enough. Heat is produced before light, but the two are nearly always found together and they are so much alike they might be called the Siamese twins.

What Heat and Light Are.—We know that both heat and light are caused by burning gases, but let’s get a little closer and find out just how and why gases which are burning send out heat and light.

Now gases are formed of particles of matter called atoms and these atoms are very small but they have a certain size and weight according to the substance they form. When these gases are cold the atoms are comparatively quiet, but when they are heated to a high temperature they are thrown into a violent state of motion and vibrate to and fro a given number of times per second, or frequency, as it is called, according to the substances they are made of.

The next question in order is what makes the particles, or atoms move, or vibrate and the answer is heat, which sometimes produces burning. Combustion, or burning is a chemical action and can be explained by saying that it is the combining of a substance with oxygen, or other substance with the production of heat and light.

The only difference between heat and light is that of wave length as we shall see presently, and the length of heat waves and of light waves depends entirely on the rapidity with which the atoms vibrate, or the frequency of vibration, as it is called. If the atoms of gas move slowly heat is sent out and if the atoms move rapidly the heat grows more intense and light is radiated.

When the atoms of a burning gas vibrate just fast enough to produce light the color of the light is red; when the atoms vibrate still faster the color of the light is green and when the atoms vibrate very fast the light sent out is violet, so we see that not only do the vibrating atoms send out light but that the rapidity, or frequency with which they vibrate makes, or determines, the color as well of the light which is sent out.

One thing more: the rapidity of the motion, or vibration, of the atoms of a gas depends entirely on the substance which is being burned, so that certain substances when burning always produce certain colors. (See Chapter XII, What the Stars Are Made Of.)

How Heat and Light Travel.—While the little motions or vibrations, of the particles, or atoms, of gas forming the flame of a candle, the Sun, and other heat and light givers could go on just the same we could not feel their heat or see their light without something or some kind of a substance which would connect them with our bodies and our eyes, like a wire connecting a push button with an electric bell. And there is something which connects us with the most distant stars and that something is called the ether.

To show how these little movements, or vibrations set up by the atoms of a flame impress us as heat or light do at a distance we will begin with a simple experiment to show what the ether is and how it acts.

If you will stand on the edge of a pool of water and throw a stone into the middle of it you will see a little ring-like ripple, or wave start out from the point where the stone struck the water; this ripple, or water wave will continue to grow larger and larger in diameter and weaker and weaker until it reaches the edge of the pond, as shown in Fig. 135, or if the pond is very large the waves will die out before they reach the edge.

Again, if you toss a number of stones into the middle of the pond one after another a series of ring-like waves will follow from the center of the pond where the stones strike the water to the edge of the pond, or until they die out.

In the same manner if a bell is struck by a blow of its tongue, ripples, or waves in the air will be sent out all around the bell. These waves in the air are called sound waves, but they are, after all, only air waves.

When a bell is struck the rim of the bell moves forth and back, first in one direction and then in the other, as shown in the diagram, Fig. 136, and we call these little movements of the bell vibrations. The movements are so small that you cannot see them, but if you put your finger on the bell you can feel them.

Although the vibrations of a bell are very small they are powerful enough to set up ripples, or waves in the air as shown in Fig. 137, and when these air waves or sound waves strike the drum of the ear it vibrates just like the bell and the auditory nerve of the ear carries the waves on to the brain and we hear the bell ring.

It must be plain now that if there was no air connecting the bell with our ears the bell might keep on ringing and yet we could not hear it.

When the air is set in motion by the vibrations of a bell, or any other device for producing from 32 to 40,000 vibrations per second, we can hear it, and when the air moves as a mass, as when the wind blows, we can feel it. Air forms a layer around the Earth that is between 200 and 300 miles thick, but out in the great space beyond there is no air. Yet the space is not empty, but it is filled instead with a substance called the ether.

Just as the air is finer than water so the ether is a million times finer than the air. It is so fine that it fills up all the little spaces between the particles or atoms of water and of the air, and it penetrates in between the atoms of the densest metals and the hardest glass, and further, and still more wonderful, it fills all of the great space in which the planets and the stars are placed.

Now when the particles, or atoms of gas are set in motion, or vibration by a flame of any kind, be it a candle or the Sun, little ripples or waves are started in the ether and if these waves are very short they affect the eye and cause the optic nerve to vibrate exactly like the vibrations which are sending out the ether waves and these waves are carried to our brains and we see the light. The way in which a flame sends out waves in the ether and is received by the eye, is shown in Fig. 138.

It takes a ripple, or wave on the water started by the impact of a stone about one-half second to travel one foot. A sound wave, set up by the vibrations of a bell, or other sound producing device, travels through the air at the rate of 1,090 feet per second, while light and heat waves set up by the vibrations of a flame, the Sun or other hot body, travel through the ether at the rate of 186,500 miles per second.

It must be plain now that if there was no ether connecting the flame, or the Sun with our eyes the flame, or Sun might continue to send out light and yet we could not see it.

How the Eye Sees.—If it was not for our ears we could not hear a bell ring nor any sound, for though the waves in the air might still be sent out we would have no means of receiving them; again if it was not for our eyes we could not see a flame, the Sun or any other source of light and, what would be worse, we could not see an object by its reflected light, for though the waves in the ether would still be sent out we would have no means of receiving them.

The eye is simply a camera on a very small scale, but what it lacks in size it makes up by the excellence of its operation. If you will set up a sheet of white cardboard on one end of your starboard, place a lighted candle at the other end and then hold your burning glass between the flame of the candle and the cardboard, as shown in Fig. 139, and do all this in an otherwise dark room, you will see a picture turned upside down, called an inverted image, of the candle flame on the cardboard.

To get a sharp picture, or image of the flame on the cardboard screen you will have to move the lens toward and away from the cardboard, and this process is called focusing. If you will fix the lens in the front of a light-tight box and place a sheet of ground glass in the back of the box you will have a simple, though crude, camera.

The eye has all of the things which the highest priced camera has and a good deal more, for all of its adjustments are automatically made, and you don’t even have to think about them.

The eye is almost as round as a ball and it can be turned a little in its bony socket in any direction. The outer part of an eye which takes the place of the box of a camera, is stretched round the whole eye like the cover of a baseball, as shown in Fig. 140. The front part of this cover forms the white of the eye, and fitting into the cover and over the lens, like a watch crystal in its rim, is the cornea, which is a tough, but transparent film and protects the iris and the lens.

Between the lens and the cornea is a thin disk, or diaphragm with a hole in its center and this is the iris; the purpose of the iris is to let in only a certain amount of light, just like the shutter of a good camera. The hole in the iris forms the pupil of the eye, and you can see the hole, or pupil, grow larger or smaller, just as the eye needs more or less light.

The lining of the eye is called the retina and this forms a screen at the back of the eye on which the light waves in the ether project the image of the object at which the eye is looking. Instead of being white like our cardboard screen the retina is very black.

The retina upon which the image is formed is connected with the optic nerve; in fact, the retina is a part of the optic nerve and is covered with a lot of little nerve ends or filaments called rods and cones.

Now when the waves in the ether sent out by the flame of a candle, or by the Sun, reach the eye, they pass through the cornea, then through the pupil, or hole in the iris, and finally through the lens which focuses the waves on the retina and forms the image there.

The different colors of light are caused by waves in the ether of different lengths; when very short waves strike the retina we say the color is violet; waves a little longer we call green and the longest waves which the eye can see form in our brains the sensation of red.

Waves in the ether which are longer than the wave-lengths the eye can see produce heat and when these waves fall on any part of the body the nerves detect them and we call the sensation heat. On the other hand waves in the ether which are too short to affect the nerves of the eye will impress a photographic plate.

The iris of the eye acts as a self-regulating shutter, which makes the hole, or pupil in front of the lens larger or smaller according to the amount of light which is needed to see an object well. If the light is strong the iris contracts, which means that the hole gets smaller and so cuts off some of the light. If the light is weak the hole gets larger and we say the pupil expands and this lets more light through the lens.

There must also be some means of adjustment to make a sharp image, or picture, on the retina, however near or far away the object may be from the eye. In a camera this is done by moving the lens and the screen closer together or farther apart and this is the purpose of the bellows of a camera.

But the eye has a much finer and quicker adjustment than this for distance. The lens is so made that the front part of it can bend just as the distance changes. You have only to look at an object and the lens is adjusted without the slightest effort or knowledge on your part.

You may wonder how light waves can pass through a substance as solid and as hard as glass or through the eye. You will remember I told you in the beginning of this chapter how ether got into every little space, even in metals and glass, as well as that it filled all the great space between the stars.

We think of glass as being very solid, and it is solid enough to keep water or air in a bottle from getting out through its pores. But glass and the substance of which the eye is made are just about as full of holes as a sieve, but the holes are so very, very small you couldn’t begin to see them even with the aid of a high-power microscope, yet they are large enough for the ether to run through just as water runs through a sieve.

When I tell you that waves in the ether which are sent out by the light of a candle or the Sun are only about 15 ten-millionths to 30 ten-millionths of an inch in length, and that the holes, or pores, in the glass and the cornea and lens of the eye, and which are full of ether, are much larger, you can readily understand that the ether waves which we call light can merrily pass through either glass or the eye and that there is nothing in the way to stop them.

To sum up briefly how the stars shine, how light travels and how the eye sees we will start with the light of the Sun and say

Reflection of Light.—The flame of a match, or a candle, an oil or a gas lamp, or Sun, comet or meteor, produces its own light, and for this reason these bodies are called self-luminous—that is, they are themselves the source of light.

All other objects which do not produce light, such as an apple, a stone, the Earth and other planets and moons are called non-luminous bodies.

Yet these non-luminous bodies can send out light if they are lighted up by some self-luminous body. It is well that this is so, or else we could never see anything that was not in itself giving out light.

If you will hold an apple or a stone in your hand and let the light of a candle or the Sun fall on it you will be able to see the apple or stone, and, although you will hardly be able to notice it, you will see them by the light which strikes them and is turned back, or reflected from their surfaces, as shown in Fig. 141.

If a rubber ball is thrown on the sidewalk it will bounce back and this is just the way light acts when it strikes most objects—it bounces back, or, to use the right word, it is reflected.

When we look at the surface of the Earth by daylight we see the sand and stones, grass and trees, houses and other objects by the light which is reflected, or thrown back from the surface of these things by the Sun.

When we look at the surface of the Earth by the light of the Moon we also see the objects by reflected light, but in this case the light is twice reflected, for moonlight is the light of the Sun falling on the Moon and which is then reflected to the Earth, where it is again reflected to our eyes from the objects it falls on. This is the reason moonlight is so pale when compared with sunlight.

Refraction of Light.—When a beam of light passes through glass, water and other transparent substances, and is bent out of a straight line it is said to be refracted.

Place a spoon in a glass of water, as shown in Fig. 142 and it will look as if the spoon is broken in two at the point where it touches the water. The bending of the beam of light will be more clearly understood from the drawing in Fig. 143.

If you will look at a star squarely through a thick piece of glass and the star seems to change its position a little you will know that the sides of the glass are not quite parallel.

A prism is a three-sided piece of glass, if we except the ends, as shown in Fig. 144. When a beam of light passes through a prism the prism affects the light in two ways: first, it bends the beam, and second, it separates the ether waves, or light waves, as they are called, according to their lengths, and as color depends on the length of the waves in the ether a prism will show the different colors on a screen and this is called the spectrum. It is shown in Fig. 145.

Lenses are pieces of glass having curved surfaces. When a beam of light passes through a lens it is also bent out of its original direction, or refracted.

A convex lens, see Fig. 146, is a lens which is thicker in the middle than it is at the edges. A convex lens is used for magnifying an object; or for forming an image so that it can be magnified by another lens as in a telescope, for forming an image on the screen of a camera, and for bringing the heat waves of the Sun to a focus, as with a burning glass.

The point where the rays of light are brought together is called the focus. You can easily find the focus of a lens by holding a sheet of paper, or the hand under the lens and letting the sunlight pass through it; where the spot of light is smallest and brightest there is the focus. The distance of this point from the lens is called the focal length.

A concave lens is a lens which is thinner at the middle than it is at the edge, as shown in Fig. 147. It is used in small telescopes and opera glasses to turn the inverted picture formed by the convex lens around so that the object can be seen in its right position, as shown in Fig. 151.

Shadows.—Shadows are useful as well as sunshine, but shadows are such common, everyday things it seems almost useless to talk about them; still you may or may not know that there are different kinds of shadows.

Of course we all know that when a candle or a gaslight or the Sun shines on an apple or any other opaque object—that is, an object that will not let the ether waves go through it—the light is cut off back of it and this dark space is called a shadow; this is also true when the Sun shines on the Earth, or on any of the other planets or their moons.

There are always two parts to the shadow of an object unless the light is a mere point or the object is very close to the screen, or surface on which the shadow falls. The dark part of the shadow is called the umbra. The edge of the object where the light and shade run together and form a partial shadow is called the penumbra and during a total eclipse this partial shadow surrounds the dark shadow, or umbra of the Earth or Moon.