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Sometimes, when walking along a road on a warın day, you may observe a curious quivering in the air just where the road seems to meet the sky, as it goes over a hill. The objects near this point appear to be distorted and to tremble, or they assume fantastic shapes. Here we have an instance of refraction caused by the heated air just above the surface of the road. The light passing through these layers of unequally-heated air is refracted unequally, and the objects that reflect the light appear distorted. In some instances the refraction may pass the critical angle, and we may see the objects apparently doubled by reflection. Warm, calm days by the sea show the same thing, when distant vessels appear repeated in the sky, or when distant land that is really below the horizon “looms” up and glimmers upon the horizon in trembling headlands. This illusion is called the mirage, and takes place when refraction exceeds the critical angle and becomes reflection.

Fill a clear glass tumbler with water, and put a spoon in it, or dip one finger in the water, and hold it above

your head so that you can look into the water from below. You will find that you cannot see through the water up into the air above. The under surface of the water will appear to shine like burnished silver, and the spoon or your finger will be reflected in it, as in a beautiful mirror. This illustrates total reflection, and shows that in this case all light thrown upward through the water is reflected from its surface. Look into the tumbler from above, and it appears full of clear water. Look into it from below, and it seems as if an opaque sheet of silver rested on the water, and shut out the view of everything above.

Take a small glass tube, and roll up a piece of colored paper or printed paper and slip it inside the tube, and then place the tube in the goblet of water. Hold the goblet in the hand near the eyes, and you can see the paper in the tube through the water. Lower the goblet till you can look down into the water from above, and the tube will appear as if made of silver, and the paper will totally disappear. To vary the experiment, lift the tube up and down in the water, and the paper will appear and disappear in the most surprising manner.

This also illustrates total reflection. The light reflected from the paper passes through the glass tube into the water, and is refracted. In certain positions the light passes the critical angle, and is reflected from the outer surface of the glass tube, and fails to reach the eye. Look into the goblet from below, and there is the colored paper pictured by total reflection on the under side of the water.

THE WATER-LENS.

Fig. 19 shows an oblong box of pine, 14 inches (35.7 centimetres) high, 64 inches (16.5 centimetres) square at the outside at each end, and made of thin

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boards, nailed or screwed together. One side is entirely open, and at the top is a round hole, 5 inches (12.7 centimetres) in diameter. On this opening rests a hemispherical glass dish, made by cutting off the round top of a glass shade. This makes a thin glass bowl, 51 inches (14 centimetres) in diameter, and it rests in the hole, partly above and partly below the top of the box.

Inside the box two strips of wood are fastened, one on each side, at an angle of forty-five degrees. On these strips rests a sheet of silvered glass, 5% inches (13.7 centimetres) wide and 816 inches (21.4 centimetres) long, or just large enough to slip into the box, as shown by the dotted lines in the picture. To keep the glass from sliding out, a tack or brad may be driven in the box at the end of the mirror.

Put the heliostat in the window, and bring a full beam of sunlight into the darkened room. Then place this box on the window-seat, or on a table next to the window, with the open side toward the window, and in such a position that the beam from the heliostat will fall on the mirror. By this arrangement the light will be reflected upward through the glass bowl. Then fill the bowl with clear water, choosing the purest and cleanest that can be found. Adjust the box carefully, and see that the beam from the heliostat strikes the mirror fully, and that the reflected beam meets the bowl on every side, so that there are no shadows inside the box.

Here we have a broad beam of light passing from the air into water, and our experiments have shown us that in such an event the light may be refracted.

Hold a sheet of paper in a horizontal position just above the bowl, and you will see that it is fully lighted up by the light thrown up by the mirror through the water. Raise the paper slowly, and the circle of light on the paper will grow smaller and brighter, till it is reduced to a small dot of intense white light.

Put a match just at this bright spot of light, on the under side of the paper, and instantly it begins to burn. Touch the lighted match to the paper, and hold the burning paper beside the bowl of water, and gently blow the smoke over the water. See what a strange cone of light appears in the smoke! It is pale below, next the water, and grows brighter and brighter till the top of the cone is reached, and here it is intensely bright. Above this cone appears another, upside down, with its point touching the point of the cone beneath it. Above, on the ceiling, is a large circle of light, perhaps several feet in diameter.

Fig. 20 represents a number of rays of light entering at the left, and reflected upward from the mirror. From our experiments we learned that light passing from the air into water, and reaching the surface in a normal direction, goes straight on through

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