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gives us sunlight, and the light from the other suns we call starlight. For convenience, we also call the reflected light from the planets starlight, and the light from our nearest planet we call moonlight.
Terrestrial light includes all the light. given out by things on the earth, or in the air that surrounds the earth. The most common light we call firelight, or the light that comes from combustion. When we light a lamp or candle, we start a curious chemical action that gives out light and heat. The result of this action is fire, and the light that comes from the flame is firelight. When a thunder-storm rises, we see the lightning leap from the clouds, and give out flashes of intensely bright light. Sometimes, at night, the northern sky is full of red or yellow light, dart
in dancing streamers, or resting in pale clouds in the dark sky. You have seen the tiny sparkles of light that spring from the cat's back when you stroke her fur in the dark, or have seen the sparks that leap from an electrical-machine. All these—the aurora, the lightning, and the electric sparks--are the same, and we call such light electric light.
Sometimes, in the night, we see shooting-stars flash across the sky. These are not stars, but masses of matter that, flying through space about the earth, strike our atmosphere and suddenly blaze with light.
The friction with the air as they dart through it is so great that these masses glow with white heat, and give out brilliant light. Two smooth white-flint pebbles, or two lumps of white sugar, if rubbed quickly together, will give out light, and this light we call the light from mechanical action.
Sailors upon the ocean sometimes see, at night, pale-yellow gleams of light in the water. A fire-fly or glow-worm imprisoned under a glass will show, in the dark, bright spots of light on his body. A piece of salted fish or chip of decayed wood will sometimes give a pale, cold light in the night; and certain chemicals, like Bologna phosphorus and compounds of sulphur, lime, strontium, and barium, if placed in the sunlight in glass vessels and then taken into the dark, will give out dull-colored lights. All these - the drops of fire in the sea, the glow-worm, the bit of decayed wood, and these chemicals—are sources of the light called phosphorescence.
These are the sources of light-the stars, the fire, electricity, friction, and phosphorescent substances. We can study the light from all of them, but the light from the sun or a lamp will be the most convenient. The light of the sun is the brightest and the cheapest light we can find, and is the best for our experiments. A good lamp is the next best thing, and
in experimenting we will use either the sun or a lamp, as happens to be most easy and convenient.
In looking out-of-doors in the daytime we find that the sunlight fills all the air, and extends as far as we can see. It shines in at the window and fills the room. Even on a cloudy day, and in rooms where the sunshine cannot enter, the light fills everything, and is all about us on every side. Now, in studying light we do not wish a great quantity. want only a slender beam, and we must bring it into a dark room, where we can see it and walk about it and examine it on every side, bend it, split it up into several beams, make it pass through glass or water, and do anything else that will illustrate the laws that
Choose a bright, sunny day, and go into a room having windows through which the sun shines. Close the shutters, curtains, and blinds, at all the windows save one.
At this window draw the curtain down till it nearly closes the window, and then cover this open space with a strip of thick wrapping-paper. Cut a hole in this paper about the size of a five-cent piece, and at once you will have a slender beam of sunlight entering the hole in the paper and falling on the floor. Close the upper part of the window with a thick shawl or blanket, and, when the room is perfectly dark, our slender beam of light will stand out clear, sharp, and bright.
As soon as we begin to study this beam of light, we find two little matters that may give us trouble. The sun does not stand still in the sky, and our beam of light keeps moving. Besides this, the beam is not level, and it is not in a convenient place. We want a horizontal beam of light, and some means of keeping it in one place all day. An instrument that will . enable us to do this, and that can be adjusted to the position of the sun in the sky at all seasons of the year
and every hour of the day, may be readily made, and will cost only a small sum of money.
On the next page are several drawings, giving different views of such an instrument and some of its separate parts. It is called a heliostat, and we shall find it of the utmost value in our experimenting in light, heat, sound, electricity, and other branches of physical science.
The first drawing represents a front-view of the heliostat. The second drawing gives an end-view, and we can now make one by simply following these few directions : The part marked A in the two drawings is a piece of pine board, 23 inches (58.4 centi