out again till it meets the ceiling, where it appears as a broad disk of light. In the diagram each ray is represented as meeting at the focus, and then all pass each other and go on in their previous directions; and you can readily see that a new cone of light will be formed, upside down, above the focus; and beyond it all the rays will spread out wider and wider the farther they go. Hold a piece of paper just above the focus, and you will see a small circle of light upon it. Raise it higher, and the circle grows larger and larger, and on the ceiling it is several feet wide.

By means of lenses of glass or water we can spread out a beam of light, gather a whole bundle of rays into a single focus-we can make distant objects appear near, make small things appear large, and large things appear small. Telescopes, microscopes, spectacles, and all kinds of optical instruments, are founded on this simple law of refraction, as shown by this bowl of water.

EXPERIMENTS IN PROJECTION.

At the optician's you can purchase a small glass plano-convex lens, 3 inches (76 millimetres) in diameter, and of a focal length of about 8 inches, for perhaps less than fifty cents. Such a lens is flat on one

side and convex on the other, and from this it takes its name. Take this lens into a room, and close the curtains at all the windows save one. Soften a piece of wax, and stick the lens into it, so that it will serve as a handle, and then hold the lens a few inches from the wall, or, if the wall is dark-colored, before a sheet of paper pinned upon the wall, and just opposite the

window. On the wall will then appear a picture of the window, and the trees, houses, and other objects that may be seen through it. Move the lens backward or forward, and you will find a place where the image on the wall becomes distinct, and gives a miniature view of the window in its natural colors, and upside down.

Here the light from the window falls upon the lens, and is refracted to a focus. This focus consists of points, each of which is formed by the convergence of rays which come from a similar point in the window. This we call a projection, because the light is projected or thrown upon the wall by the lens. To understand this we must notice that every part of the window sends light into the room in every direction. Every part sends light into the entire lens, and each beam is refracted and takes a new direction be

FIG. 22.

yond it, so that, ultimately, all the rays meet at the focus. In examining this projection, you will notice that the glass is quite near the wall when the focus is clear and sharp. If we measure the distance from the lens to the projection, we shall get a certain measurement. This measure we call the focal distance of the lens.

Fig. 22 represents two rays from the top of the window and two from the bottom, and shows the

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