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pasteboard disk under them, to hold them in place. Now, if you hold the top upright in a plate and make it spin, you will see a beautiful ring of green color round the spinning top.

When it stops, take off the pasteboard, and revolve the colored disks one on the other, so that half of the red and half of the green can be seen. Now spin them on the top, and instantly you have a ring of yellow. Move the disks again, so as to display onequarter of the green and three-quarters of the red, and when the top spins you get a deep-orange ring. Move them again, and let the green hide nearly all the red, and the top shows a greenish-yellow ring. In this manner you may mix the red and green in greater or less proportions, and the ring of color on the top will exhibit new shades of yellow with every change.

In the same way, combine the green disk and the violet, and the spinning top will show a new shade of blue with every proportion in which the green and violet are mixed. Put on the red and violet disks, and let each show more or less, and shades of purple will be shown. Put on all three disks-the red, green, and violet-and arrange them so that one-third of each is shown, and the ring will be gray. Change the proportions, and you will see each time new shades of gray or white.

This is a very simple toy, but it serves to show how these three colors may be combined to produce every color in the solar spectrum. The color will vary very greatly, and new and beautiful shades of yellow, blue, purple, and gray, will be found at every trial. Red, green, and violet, may be tinted with other colors in the most charming manner, and hours can be filled with amusement and instruction by experimenting with this color-top and its ever-changing colored rings.

To exhibit the colored rings on this top before a number of people, make a disk of stiff cardboard about 5 inches (12.7 centimetres) in diameter, and cut out three holes at equal distances from each other near the edge. Over these holes place pieces of red, green, and violet, or ultramarine-blue glass, one color on each hole, and fasten them down with little bands of paper at the edges, and secured with mucilage. Place this disk on the color-top, and hold it upside down just above the large lens in the water-lantern. Have the lantern prepared to give projections on the screen (see section on water-lens), and then you will see three spots of colored light on the screen, and, by making the top spin round on the handle by means of the string, the three spots of color will whirl round in a ring, and, if the top moves fast enough, we shall see

a ring of white or gray. Cover the violet glass so as to shut out all the light, and then make the top spin, and the two spots of red and green will appear on the screen in the form of a yellow ring. In this manner all the effects exhibited by the color-top may be projected on a large scale on the screen, and make a most interesting and beautiful exhibition that will be sure to please all who see it.

DIRECT RECOMPOSITION OF THE COLORS OF THE SPECTRUM.

Let the spectrum fall on a mirror, and throw its reflection upon a distant part of the room. Procure a slip of looking-glass half an inch wide and about three inches long, and place this on the mirror in any color of the spectrum. By tilting the slip of lookingglass, any color can be thrown on to any other color of the spectrum, and thus an endless variety of colors can be formed by compounding their elementary component colors.

EXPERIMENTS IN REFLECTED COLORS.

Fig. 29 represents a flat block of wood having a short stick set up at one corner. On this stick is fastened, by means of a lump of wax, a strip of clear window glass about 1 inch (25 millimetres) wide,

and 3 inches (7.5 centimetres) long. Just behind the stick is another piece of glass of the same size fast ened to the block of wood by a mass of wax. Place the instrument on a table near a window, and then sit before it with your back to the light. Cut out small

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bits of paper and paint one red-vermilion, another emerald-green, and the third violet, as in our experiments with the color-top. Then place one of these in front of the instrument, at the spot marked in the drawing, and, sitting close to the instrument, look down into the glass on the stick and you will see the bit of colored paper reflected in the glass. Suppose this is the red piece. the green piece at the ring marked A.

Then place

On looking

into the glass you can now see both the green piece reflected in the glass and the red behind it. While thus looking at both, move the one or the other till they appear to come in line, or one over the other, and then, in place of seeing a red and a green piece, you will see a single yellow piece.

Again we have a combination of colors, and we can place the red and violet or the violet and green pieces before and behind the glass, and see the colors combine precisely as in the color-top. If it is not convenient to make this instrument, these effects can be shown with a piece of clear window-glass, by simply holding it in the hand so that one color can be seen reflected in the glass, and the other directly through it.

With the instrument we can combine all three colors by placing them in the positions marked A, B, and C, and then looking through both glasses at once. The color at A will be seen through both glasses, the color at C will be seen in the upper glass and in a line with the first, and the color at B will be seen reflected on the surface of the lower glass; and, if all three are in the right places, we shall see only one piece, and that will be white or gray.

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