CHAPTER II.

Light.

Color.

In what way is the sun useful to us?

It gives heat, which makes us warm, and it gives light, so that we can see.

Is it useful in any other way?

It gives light and warmth to all animals as well as to all plants.

What make the flowers and leaves so beautiful?

The light and heat from the sun make them beautiful.

What do the light and heat from the sun ripen?

They ripen the apples, cherries, and other fruits; also, the wheat, corn, and other grains, and make them fit for food.

When it is night-time or dark, can we distinguish objects?

No; because all things are then without color, and they cannot be seen.

As it grows light in the morning, what takes place?

At first, we see the objects around us faintly; but as it grows lighter, we see them more distinctly.

Do they all show the same color in the light?

They do not; they show different colors.

What enables us to see the color of different things?

Light enables us to see the color of everything.

What kinds of light have we?

We have the light of the sun, moon, and stars; also, the light from the burning of any substance.

Does the light show many colors?

It does show many colors or shades of color.

How may some colors be made?

They may be made by mixing other colors together.

How can we make a purple color?

By mixing a red and a blue color together.

How can a green color be made?

By mixing a blue and a yellow color together.

How can an orange color be made?

By mixing a red and a yellow color together.

How does the light come from the sun?

It comes in rays or straight lines.

What may be done with a ray of light?

It may be separated into different colors.

Into how many colors may a ray of light be separated?

Into _seven_ colors, like the colors of the rainbow.

What names are given to these colors?

Red, orange, yellow, green, blue, indigo, violet.

How may these colors be separated?

By a prism, or three-sided piece of glass.

How may this be done?

If a sunbeam, shining through a hole in the window-shutter of a dark room, should fall upon a prism, it will be broken up as it passes through the prism, and be shown on the opposite wall, in the seven rainbow colors.

In Fig. 6, D is a sunbeam passing through a shutter. When it passes through the prism, at E, it is separated into the seven rainbow colors, as is shown on the opposite wall.

In what order will these colors appear?

They will always appear one above the other, in the order named.

How may this order be remembered?

By the word Roy-g-biv, made from their initial letters, beginning at the bottom.

From what are the colors of the rainbow made?

From the rays of light coming from the sun.

When is a rainbow seen?

A rainbow is seen when the sun shines through the drops of water as they fall through the air.

How do the drops of water help to make a rainbow?

The drops of water act like little prisms, and separate the rays of light passing through them into the colors of the rainbow.

What two things, then, are needed to make a rainbow?

A shower of water, and a bright sunshine.

In what part of the sky does the rainbow appear?

In the part opposite to that in which the sun is, at the time of the shower.

How does the rainbow appear?

It appears like a great arch, spanning the sky, and its splendid bright colors are very beautiful.

What may we remember in reference to the rainbow?

We may remember that God set his “bow” in the cloud, as a token that the world should no more be destroyed by a flood of waters.

Where else may we see a variety of colors?

In the soap-bubble.

Why are these colors constantly changing?

Because the walls of the bubble are constantly growing thinner, and different colors are, therefore, reflected from them.

Why do the walls of the soap-bubble grow thinner?

Because the water runs to the bottom of the bubble until its top becomes so thin as to burst.

Can we tell the color of any object by touching it?

No; we must see it in order to tell its color.

How do we know this?

If there are two coats, one blue and the other black, we cannot tell which is the blue one or which is the black one, by feeling them.

What is necessary in order to see the colors?

Light is necessary; because all colors come from the rays of light.

Can we see the different colors in the ray itself?

We cannot; because God has so blended them together that they cannot be seen by us.

When most of the rays of light pass through a substance, what is said of it?

The substance is said to be _transparent_.

Name some transparent substances.

Glass, ice, diamonds, air, and clear water.

When only a few rays of light pass through a substance, what is said of it?

The substance is said to be _translucent_.

Name some translucent substances.

Flint, isinglass, scraped-horn, and china-ware.

When no rays of light pass through a substance, what is said of it?

The substance is said to be _opaque_.

Name some opaque substances.

Wood, iron, coal, and granite.

Can we see through a glass window?

Yes; because the glass is transparent, and the light passes through it.

Can we see through a looking-glass?

No; because the back of the looking-glass is covered with quicksilver, which prevents the rays of light from passing through it.

What becomes of the light falling on the looking-glass?

It is thrown back from the glass.

What is this called?

It is called _reflection_.

What objects are good reflectors of light?

Those having smooth and polished surfaces; such as tin, silver, gold, and quicksilver.

What objects are poor reflectors of light?

Those having dull, uneven surfaces; such as iron, wood, cloth, leather, and calico.

Of what is every ray of sunlight composed?

It is composed of the seven colors of the rainbow.

What does a looking-glass do with these colors?

A looking-glass throws them all back from its surface.

What does a glass window do with these colors?

A glass window permits all these colors to pass through it.

What do opaque substances do with these colors?

They throw back or reflect some of them.

Do they all reflect the same color?

No; some reflect one color, and some another color.

What color does the grass reflect?

It reflects the green color; therefore, the grass is green.

What does the grass do with the other colors?

It absorbs or hides them in itself.

How do we know the color of anything?

We know it by the color which it reflects.

Why is the rose red?

Because it reflects the red color, and absorbs or hides the other colors in itself.

When anything _absorbs_ all the colors of a ray of light, what color is it?

It is black.

When it _reflects_ all the colors of light, what color is it?

It is white.

Why are the letters on this page black?

Because they absorb all the colors of light, and reflect none.

Why is the paper white?

Because it reflects all the colors of light, and absorbs none.

Why are all things black in the dark?

Because there are no rays of light falling on them; hence, there is no color for them to reflect.

Does an object reflect its color in every direction?

It does reflect its color in every direction.

How do we know this?

Because thousands of persons, in different directions from the same object, can see it at the same time.

How are the rays of color reflected from an object?

They are reflected in straight lines.

Where must a person be so as to see an object?

He must be in a position so that there is nothing to prevent the rays of light passing in a straight line from the object to his eyes.

What color does a red ribbon reflect?

It reflects a red color.

Why are some red ribbons of a brighter red than others?

When they reflect a great many red rays, they are of a bright red color; but when they reflect only a few red rays, they are of a dull red color.

Upon what does the brightness or dullness of a color depend?

It depends upon the number of rays of that color reflected.

What variety does this produce?

It produces a great variety in the shades and tints of color.

Where may many of these shades and tints be seen?

In the pretty flowers that grow in the woods, fields, and gardens; in the beautiful tints of the leaves in autumn; and in the plumage of the birds that visit us every spring, and cheer us with their songs.

What may these things teach us?

They may teach us the goodness of God, in giving us a world so full of beauty; and we should feel ever thankful to Him for the great pleasure that we have in seeing them.

Why cannot we see the light of a lamp or candle distinctly, in the day-time?

Because the strong rays from the sun hide the more feeble rays from the lamp.

Why cannot we see the stars in the day-time?

Because the bright light from the sun hides their more feeble light.

When can we see the glow-worms or fire-flies best?

At night, when it is dark; because, at other times, the light of the sun hides their feeble light.

Where may we see them at night?

Flying about in all directions, looking like bits of fire moving in the air.

Does a looking-glass reflect all the colors of light falling upon it?

It does; hence, the ray of light is not changed.

When the object is of a blue color, why is blue only reflected?

Because it is the only color falling upon the glass, and there is no other color to reflect.

Is the color of any object changed by falling upon a looking-glass?

No; the color of every object is reflected from a looking-glass without change.

Do other objects generally reflect the colors falling upon them?

They do not; they absorb or hide the color of all other objects, and reflect only their own color.

If every object was a good reflector of color, like a looking-glass, what would result?

Then every object would reflect the color of every other object around it, and there would be an endless confusion of colors.

What objects reflect the most light?

Those of a white or light color.

What objects reflect the least light?

Those of a darker color; and the nearer the color approaches to black, the less light it reflects.

Why is it darker when the sky is covered by dark-colored, than by light-colored, clouds?

Because the dark-colored clouds reflect less light, than the light-colored clouds do.

What advantage do we derive from the green color of the grass and the leaves?

They reflect a soft, pleasant light, which does not dazzle or hurt our eyes.

How would it be if those objects were white?

They would dazzle us by the amount of light reflected.

If we wish to make a room light, of what color should the walls be?

They should be white; and when papered, it should be with light-colored paper.

Why does dark-colored paper darken a room?

Because it absorbs some of the rays of light which come in at the windows; hence, there is less light in the room. Dark-colored carpets, or dark furniture, darken the room for the same reason.

Does the light from a burning body differ from sun-light?

Yes; it differs in intensity, in color, and in many other ways.

What effect does the lamp-light have upon the color of some substances?

It changes their color.

Where may we find an example of this?

In trying to match the colors of thread or silk at night.

Why is the color different by lamp-light from what it is by daylight?

Because the yellow color of the lamp’s rays acts upon the color of the silk, forming a third color. Blue silk becomes green by lamp-light, and red silk becomes orange.

Are all persons able to distinguish one color from another?

No; some are color-blind. To such persons all colors appear to be alike.

What curious instances of color-blindness may be mentioned?

Dr. Mitchell tells of an officer who bought a blue coat and a red waistcoat, thinking them to be of the same color. He also tells of a tailor who patched a black waistcoat with a piece of crimson; and of another, who put a red collar on a blue coat. Dr. Dalton lost a piece of red sealing-wax in the grass, and he could not find it because it and the grass seemed of the same color to him.

Refraction of Light.

How do the rays of light from the sun come to us?

They come to us in straight lines.

When the rays of light pass from one substance to another more dense, what takes place?

They are bent or turned from a straight course.

What is said of these bent rays?

They are said to be _refracted_.

What is meant by refraction?

Bending a ray of light, as it passes from one substance to another of different density; as from air to water, or from water to air.

Where may we see an example of refraction?

In the rays of light from a rod which has one end in the water.

Why does the rod appear to be bent?

Because light coming from the part of the rod under the water, is refracted or bent at the surface of the water, while the light from the part above the surface, comes in straight lines to the eye; hence, the rod appears to be bent at the surface, as is represented in Fig. 7.

How does a pole appear when one end is placed in the water?

It appears to be bent where it meets the surface of the water.

How does the part of the rod under water appear?

The part under the water appears to be raised up, as may be seen in Fig. 7.

How does the bottom of a pond or stream of water appear?

It appears to be raised up, and the water does not seem so deep as it really is.

Why is this the case?

Because the light from the bottom of the pond is refracted or bent at the surface of the water; hence, the bottom appears to be raised up.

How much does the bottom of a pond or stream of water seem to be raised up?

About one-third of the depth of the water.

If the water is really six feet deep, how deep does it appear?

It appears to be only four feet deep.

What has frequently happened from not knowing this fact?

People have been drowned from getting into water deeper than it appeared to be.

Do the fish and other things floating in the water appear to be raised up?

Yes; like the bottom of the stream, they also appear to be only two-thirds as far from the surface as they really are.

If we wish to spear the fish, what must we do?

We must aim below the place where they seem to be, or we must strike perpendicularly at them.

Reflection of Light.

When a ball is thrown against a wall, what is the path through which it goes called?

The line of incidence.

When the ball bounds back, what is its path called?

The line of reflection.

What is the angle at which a ball strikes the wall called?

The angle of incidence.

What is the angle at which the ball rebounds called?

The angle of reflection.

When a ray of light falls upon a looking-glass, what is the path through which it goes called?

The line of incidence.

When the ray is reflected, what is its path called?

The line of reflection.

The _angle of incidence_ is the angle between a perpendicular and the line of incidence; and the _angle of reflection_ is the angle between the perpendicular and the line of reflection. The two angles are always equal to each other, as is represented in Fig. 8.

When a ray of light strikes a looking-glass obliquely or slantingly, what is done with it?

The ray is reflected as obliquely from the glass as it strikes or falls upon it.

Why is the reflection of the sun in a pond of water seen near the edge at noon?

Because the angle of reflection is equal to the angle of incidence; and the observer must be nearly over the place where the rays strike, so that, when reflected, they may meet his eye.

Where is the reflection seen in the morning or in the evening?

It is seen nearer the middle of the pond; because the sun’s rays then fell more obliquely on the pond, and are reflected more obliquely to the eye.

When an opaque object is placed between a candle and a wall, why will there be a shadow?

Because the opaque object prevents the rays of light from falling on the wall.

Why will the shadow be larger, the nearer the object is to the candle?

Because the rays of light diverge, in every direction from the candle, in straight lines, and the nearer the object, the more rays will it intercept, as is represented in Fig. 9.

In Fig. 9, let L represent the flame of a candle. When the object is placed at A, the shadow on the wall will extend from C to D; but when the object is moved nearer, as at B, the shadow will extend from E to F.

Why does an object seem to be smaller, the farther it is from us?

Because the angle at which the light from it strikes the eye, is less when at a distance, than when near to the eye.

In Fig. 10, the diameter of the near circle is from A to B; but the diameter of the more distant circle, although just as large, seems only to be from C to D.

Why does an object grow more dim the farther it is from us?

Because the rays of light from it spread out in every direction; and fewer rays enter the eye when it is at a distance than when it is near by.

What is the use of the telescope?

It is used to view distant objects.

Why can we see a distant object more distinctly with a telescope, than without one?

Because the telescope enlarges the image of the object, and it also collects more light from that object than is collected by the unaided eye; thus, making the image distinctly visible.

What class of objects do we view with a telescope?

We view the heavenly bodies; the sun, moon, and stars.

What is a spy-glass?

It is a glass mostly used to view distant objects on the earth.

Is the spy-glass like the telescope?

The spy-glass is much like the telescope, only it is smaller.

When looking through a spy-glass, how do objects appear?

They seem to be brought nearer to us, and they appear larger.

By whom are spy-glasses much used?

Spy-glasses are much used by persons on board of vessels, while at sea.

What instruments have we for magnifying small objects?

We have magnifying glasses and microscopes.

What are magnifying glasses?

They are single glasses or lenses, used for magnifying small objects.

By whom are magnifying glasses used?

They are used by jewellers, watchmakers, engravers, and others who examine small objects.

What are microscopes?

They are instruments used for examining very small objects.

How does a drop of rain-water or vinegar appear under the microscope?

It appears to be full of strange-looking creatures, which are always in motion.

How do the smallest insects appear?

They appear to be as perfectly formed as the larger ones which are seen with the naked eye.

What may be seen with a microscope?

Thousands of things too small to be seen by us unless they are magnified.

What kind of spectacles do near-sighted people use?

Near-sighted people use glasses, thickest at the edge and thinnest at the centre.

What kind of spectacles do we use as we grow old?

As we grow old we use glasses, thinnest at the edge and thickest at the centre.

Of what use are spectacles?

When the vision is defective, spectacles enable us to see objects more clearly.

How should spectacles be made?

They should be so made as to render objects distinct, but neither to magnify nor diminish them.

As people grow old, how do their eyes change?

Their eyes change so that they see objects at a distance better than those which are nearer, hence, in reading they hold a book farther from the eye.

The Eye.

What is that part of the eye called, which is blue, gray, or brown, in different persons?

It is called the _iris_.

What is the pupil of the eye?

The pupil is the black spot or hole in the centre of the iris.

What is the use of the pupil?

The pupil is the window of the eye, and the rays of light which enter the eye must pass through it.

What power has the iris over the pupil?

The iris has the power of making the pupil larger or smaller, according to the amount of light which it receives.

How does a bright light affect the iris?

It causes the iris to contract, so that the pupil becomes smaller.

How does a faint light affect the iris?

It causes the iris to dilate, so that the pupil becomes larger.

Of what advantage is the iris to the eye?

The iris acts like a sentinel to protect the eye from any sudden light.

How does a sudden light affect the eye?

A sudden light causes pain to the eye.

Of what shape is the pupil of the eye?

In man, the pupil of the eye is circular.

What is the size of the pupil of the eye?

The pupil varies from the one-twentieth to the one-third of an inch in diameter, depending upon the brightness of the light entering the eye.

How else are our eyes protected from any sudden light?

By means of our eyelids, which may be closed, and thus shut out all the light.

When we pass from a well-lighted room into the open air, at night, why does it seem darker at first than it does afterwards?

Because the pupil is contracted at first; but it soon dilates, and allows more rays of light to pass into the eye, so that the night seems less dark.

Why does the pupil become larger in the twilight?

So that more rays of light may enter the eye, whereby objects may be more distinctly seen.

Why can an owl or a bat see at night?

Because the pupil of the eye in them is quite large, and admits much light.

Why do they stay in dark places through the day?

Because the bright light of day hurts their eyes.

May the image of an object be retained in the eye after the eyelids are closed?

It may for a very short time, as any one can prove by first looking at some bright object, and then closing the eyelids.

Why does a burning coal moved rapidly around, seem like a circle of fire?

Because the light from it is retained a short time by the eye, thus seeming to form a complete circle.

Why cannot we count the posts in a fence, when riding rapidly in a car?

Because the light from each post falls in such quick succession upon the eye, that it cannot distinguish one post from another.

Can objects be seen distinctly when placed near the eye?

No; when objects are within six inches of the eye they cannot be seen distinctly.

Can objects be seen when at a great distance from the eye?

Yes; objects may be seen even when millions of miles away from us; thus, the sun is seen although ninety-five millions of miles from the earth, and some of the stars which we behold in the heavens, are still farther away than the sun.

Of what use is the eye to us?

The eye gives us ideas of the size, the shape, the color, the place, the distance, and the movements of things around us, so that we can use them the better to promote our comfort and our happiness.

What may we remember about the light?

And God said, “Let there be light;” and there was light. Thus the day was separated from the night, and the sun was made to send forth his silvery rays upon hill and valley, field and forest, causing the rarest buds and most beautiful blossoms to come forth from the lifeless earth, although no eye, save the All-Seeing One, was there to behold them!