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path they take. To draw every ray would confuse the picture, and by examining these four we can form an idea how they all travel together in a crowd and meet beyond the lens in new positions, and all closely drawn together in a focus. Those from the top of the window are refracted in one direction, those from the bottom in another, and thus they cross each other, and the projected image of the window appears to be upside down.
The picture on page 77 in this section represents the heliostat in position in a dark room. On a table in front of the instrument is the plano-convex lens, mounted on a lump of wax, fastened to a block of wood, and placed with the convex side toward the sun. The opening of the heliostat is covered by a piece of smoked glass, having a figure of an arrow drawn upon it. The light passes through the glass where the smoke was brushed away in drawing the arrow, and falls upon the lens. By refraction the beams of light form an image of the arrow upon a white screen.
This screen is made of white cotton cloth, and is hung about 15 feet (4.57 metres) from the lens. The result is a large projection of the arrow, upside down, and in white on a black ground. Move the lens backward or forward slightly, and you will find a place where the projection is sharp and clear, and then the lens may be fixed there while we project other images on the screen.
This simple and inexpensive apparatus thus makes an excellent magic lantern. Common painted or photographic lantern - slides may be placed upside down at the opening of the heliostat, and will be projected on the screen clearly and distinctly, as with the best magic lanterns. Concerning the use of this lantern, and the slides that may be used in it, more may be found under the section on the water-lantern.
If it is not convenient to use a heliostat, this lantern may be used by taking a beam of sunlight, as it enters through a hole—4 inches (10 centimetres) in diameter—in the shutter, and reflecting it in a horizontal direction through the lens by means of a handmirror.
THE FOUNTAIN OF FIRE. Fig. 23 represents a flat-bottomed flask, used by chemists. It has a narrow neck at the top, a flat base, and a hole at the side. This hole may be cut in the flask by means of a tube of brass one-quarter inch in diameter. This tube has a square end which is scored by two or more cross-cuts with a Vshaped file. A block of wood, having a hole of onequarter inch in diameter, is placed against the flask; and then the tube, armed with emery and water, is inserted in this hole, and by twirling the tube in the fingers the hole in the glass is made. The tube may also be put in a lathe. In the picture the flask stands upon a shelf in front of the heliostat, and
just beneath it on the floor is placed a tub or a water-pail. These few things and some pieces of colored glass will enable us to perform a most interesting and beautiful experiment both in refraction and reflection. Place the finger over the hole in the side of the flask and fill it with water. Place the flask on the shelf so that the beam of light from the heliostat will strike the glass opposite the hole in the side.
Look at the beautiful cone of light in the water. The beam of light is refracted and brought to a focus as in our other experiments, except that here the cone is entirely under water. Study this singular cone carefully, and adjust the flask so that the point of the cone shines on the finger at the hole in the side. When this is done, make the room as dark as possible, and then remove the finger and let the water fall in a stream into the tub on the floor.
How magical! The curving stream of water is full of light, and appears like a stream of molten iron. The spot where it falls seems touched with fire. Put your finger in the stream of water, and it is brightly illuminated. Of course, the water soon runs down, and the display stops. To prevent this, bring water in a rubber tube from the water-pipes in the house, and then regulate the supply so that the receiver may be kept full as fast as the water runs out.
Place a piece of red glass behind the flask in the beam of sunlight, and the stream of water will look like blood. Touch it, and the hand will be crimson, and the scattered drops that fall in a shower into the tub will shine like drops of red fire. Place a green or blue glass behind the flask, and the stream of water will turn green or blue, and present a most singular appearance. Hold a goblet in the stream, and it will overflow with liquid light. Flashes and sparkles of fire will appear in it, and foam over the sides, shining with brilliant light.
This beautiful experiment is as interesting as it is strange and magical, and it illustrates both refraction and total reflection. The flask makes a lens, and the falling stream of water is lighted up by the cone of light that enters it at the hole in the flask. Both the water and the light pass out of the hole together, the light inside of the water. That this is so, may be proved by permitting the water to escape, when the light will be seen shining out of the hole horizontally into the room. Why, then, does it not shine out into the room while the water is escaping? When the stream of water is flowing out, it falls in a curve into the tub on the floor. The beam of light, passing out with the water, meets its curved surface at such an angle that it is totally reflected. This beam of reflection again meets the surface of the water, and is again totally reflected. In this manner