SEEING THE STARS
=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_.
[Illustration: FIG. 135.—RIPPLES OR WAVES ON WATER.]
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.
[Illustration: FIG. 136.—VIBRATION OF A BELL.]
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.
[Illustration: FIG. 137.—SOUND WAVES IN THE AIR SET UP BY BELL.]
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.
[Illustration: FIG. 138.—WAVES IN THE ETHER.]
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.
[Illustration: FIG. 139.—FORMING AN IMAGE WITH A LENS.]
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_.
[Illustration: FIG. 140.—THE HUMAN EYE.]
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
(1) That the Sun is made up largely of gases, and that (2) These gases
are formed of various substances, and that (3) The gases produce
terrific heat, which means that (4) The atoms or particles which form
the gases are in violent motion or vibration. (5) These vibrations
start out waves in the ether which travel out into space at a speed
of about 186,500 miles per second. (6) On reaching the eye the waves
pass through the lens and form a picture or image (7) On the retina, or
screen of the eye, which is made up of the ends of nerves, and these
vibrate just as the atoms of the gases in the Sun which sent out the
waves vibrated, and finally (8) These nerve vibrations are sent over
the optic nerve to the brain, where they take on the shape, size and
colors of the Sun.
=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.
[Illustration: FIG. 141.—LIGHT REFLECTED BY AN APPLE.]
[Illustration: FIG. 142.—LIGHT REFRACTED. SPOON IN GLASS OF WATER.]
=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.
[Illustration: FIG. 143.—HOW LIGHT IS REFRACTED.]
[Illustration: FIG. 144.—PRISM.]
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_.
[Illustration: FIG. 145.—PRISM FORMING A SPECTRUM.]
[Illustration: FIG. 146.—CONVEX LENS.]
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.
[Illustration: FIG. 147.—CONCAVE LENS.]
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.