THE WATER-LENS.

Fig. 19 shows an oblong box of pine, 14 inches (35.7 centimetres) high, 6 inches (16.5 centimetres) square at the outside at each end, and made of thin

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, 5 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 8 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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the water in the same path. If it enters the water on either side of this normal, it is refracted or

turned aside, and takes a new path. The greater the angle at which it enters the water, the greater the re

fraction. In the diagram the line in the middle represents the ray of light in the centre that meets the water at a normal, and passes straight through it and on into the air above. On either side the rays are represented as refracted, or bent out of their track, and obliged to take new paths. The greater the distance of the ray from the normal, the greater its refraction. Now, as all the rays at the same distance from the normal are refracted to the same degree, it follows that there must be a place where all these rays of refraction will meet.

Look at the cone of light over the bowl of water, and you can see the spot where all the rays of light are concentrated. Here they meet in what is called a focus. You can readily remember this word, because it means a hearth, or burning-place, and we saw our match take fire just at that point. The rays of sunlight contain heat as well as light, and, if we gather them altogether in a bundle, of course we shall concentrate both the heat and light. A bit of paper held in the focus glows with intensely white light, and presently begins to smoke and burn in the concentrated heat.

This bowl of water is called a lens, and, by means of refraction, we may use it to concentrate light and heat. Beyond the focus you observe the light spreads