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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

Fig. 29.

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 C 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. Then place the green piece at the ring marked A. 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.


Cut out three pieces of drawing-paper about 2 inches (5.1 centimetres) square, and paint one red, another green, and another violet, using the paints we bought for the color-top. If these are not at hand, cut out squares of red, green, and violet paper, and squares of yellow, pink, blue, or any other colors you can obtain. Lay a piece of black cloth on a table, near the window, and then sit before it with your back to the light, and place the red square of paper on the cloth. Take a sheet of white or, still better, lightgray blotting-paper in the right hand, and hold it just above the red square in such a position that it can be quickly slipped over it, so as to hide it from sight. Now look steadily at the red square for a minute or two, and then slip the gray paper over it. In a few seconds there will appear on the gray paper a curious image just the size and shape of the red square, but of a bluish-green color. It will grow brighter quickly, and then fade away, leaving nothing but the gray paper. Put the green square on the black ground, and, after looking at it for a moment, cover it with the gray paper, and a pink image of the square will seem to shine out of the gray paper for a moment, and then fade away. Look at the violet square, and then suddenly hide it, and a pale, greenish-yellow image will be seen. Get a square of yellow paper, and repeat the experiment, and you will see a violetblue image. In the same way try an orange square, and get a violet image; try greenish-yellow, and get a pink image.

These strange, ghostly after-images that linger after any color has been suddenly removed, result from an action in the eye. Having looked at red till the eye becomes weary, and then having suddenly taken the red away and replaced it by a white surface, the nerves of the eye send us another sensation that we call bluish-green. The nerves sensitive to red having become fatigued, the nerves sensitive to green and violet are fresh and fully sensitive to the green and violet rays in the white light reflected from the paper.

Every color gives a particular after-image, and this image is always of a color that is said to be complementary to it. Red is complementary to bluish-green, orange to sky-blue, yellow to violet-blue, green to pink, and so on through all the colors.

To vary this experiment, you may take a small bit of green paper, say about the size of a wafer, and lay it on the middle of a square of orange-colored paper, and then, after looking at the two colors for a moment, hide them both with the gray paper, and the

after-image will be blue, with an orange spot in the centre. Take off the green wafer and put on a blue one, and make the experiment. Put a large spot of ink in the centre of this orange square, and the blue after-image will have a gray spot in the centre. Black, we know, is the absence of color, and on looking at the black spot the eye is not excited at all, and the blue image appears to have a hole in the middle through which we see the gray paper. If we put a white wafer on the orange square, we shall see a blue after-image with an orange spot.

These strange and curious after-images, appearing like colored ghosts on the gray paper, may afford both amusement and instruction, and will give us the complementary color of any color we use in the experiments. These complementary colors, when placed side by side, always give the eye a pleasing sensation, and we say that the colors look well together.

Take a piece of red cloth or paper and hang it up before a white screen, or upon a white wall, in a dark

Stand near the window and look at the red cloth, and you will not be able to see it. Then slowly open the window-shutter, and permit a little light to enter the room. Now the red cloth looks like a black patch on a gray wall. Let in a little more light, and it turns to a deep, dark red. Gradu


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