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After all the cards have been dealt, any top card can be used in building. The top card on any corner or side pile may be placed on any other of those piles whose top card is just above or just below it in rank, and of the same suit. The cards may be examined at any time. The cards in the side and corner piles may be twice redealt. If, after they have been played the third time, the piles on the Foundation cards can be completed, the player has won; otherwise, he has been defeated.

EYES, Experiments with the. The eyes are described in C.C.T.

1. Hold up the forefinger about a foot from the face, and look at an object beyond it, a tree for instance. The forefinger will appear double. Then look at the forefinger, and the tree will appear double. The reason is that when the two eyes are looking at the forefinger the right eye sees the tree on the right side of the finger, and the left eye sees it on the left side. When they are both looking straight at the tree, each sees the forefinger in a different place. If one eye be covered it is impossible to see either forefinger or tree double.

2. Place two bits of white paper on a table, about two feet apart. Cover the left eye, and with the right look steadily at the left piece of paper, at the same time walking slowly backward. A snot will be found where the ngnt nanrt bit ot paper will disappear. By looking with the left eye at the right hand bit, the left hand bit can be made to vanish in like manner. By moving the head ever so little forward or backward the bit of paper will be made to appear again. The nearer the pieces are together the nearer the eye has to be placed to them to make one disappear. If, instead of bits of paper on a table, pencil dots two inches apart on a sheet of paper be tried in the same way, one will vanish when the paper is held about six inches from the eye. In each case

the reason is that the retina of every person's eye has a blind spot in it, and when the image of the paper or pencil dot falls directly on that spot, it cannot be seen.

3. Hold the eye two or three inches from the perpendicular edge of some object seen against a bright background, part of a window sash, for instance, or, if it be night, a ruler leaning against the shade of a lighted lamp. Shut one eye, and holding the edge of a sheet of paper close to the other move the paper to and fro. The edge of the object will seem to move out to meet it. Repeat the same thing, standing about twenty feet away from the window sash or ruler, and the edge will appear to shrink away from the paper.

4. Let one person hold a candle, lamp, or some other bright object in front of another's eye. He will see in the eye three reflections. One is from the outside of the eyeball, another from one surface of the lens inside the eye, and the third from the other surface of the lens.

5. Cut out of black paper two exactly similar figures, crosses for instance, and place them side by side, almost touching, on a sheet of white paper. Hold them about three inches in front of the eyes, and three figures will be seen instead of two. The middle one consists of two, the image of the right hand figure, as seen by the right eye, being added to that of the left hand figure as seen by the left eye.

6. To see stereoscope pictures without a stereoscope. The stereoscope is described in C. C. T. Hold a stereoscope picture before the eyes and by fixing them as if to look at a distant object make the picture appear double, as in Experiment 1. With practice, the eyes can be so controlled that the two pictures nearest each other can be made to overlap and melt into one, in which objects will stand out just as when seen through the stereoscope.

7. Place a scrap of colored paper or cloth on a gray ground, and look steadily at it for about a minute. Snatch the scrap away and in its place will be seen a spot of exactly the same shape but a different color. If the scrap is green, the spot will be red, which is the complementary or opposite color to green; if yellow the spot will be violet. If, instead of

Fig. 1.—Experiment 7.

pulling the paper away, the eye be directed to the ceiling, the spot will be seen there. These spots, which are often called "ghosts," are caused by the action of light on the retina. The accompanying figure (Fig. 1) is a good one to experiment on. Look at it steadily for some time and then look at the ceiling, where it will shortly appear in black on a white ground.

8. Light a splinter of wood, and whirl it about in a dark room. It will seem like a circle of fire. This is because the image of the lighted end remains in the eye while it is being twirled around. For other experiments, showing that images remain in the eye for a fraction of a second, see Thaumatrope, Zoetrope, and Chameleon Top.

9. In a room in which there is no other light, hold a candle before one eye, closing the other. The candle must be moved up and down a little on one side of the eye and two or three inches from it. Presently there

will appear black shadows on a reddish ground, looking somewhat like leafless trees. These are the shadows of the blood-vessels on the retina.

10. Hold a pin so near the eye that it appears quite blurred. Look at it in the same position through a pinhole in a piece of paper, and it will be seen distinctly. In this way a pinhole in paper may be used to look at other small objects. It does not magnify them, but enables us to hold them much closer to the eye than we otherwise could.

11. Roll up a sheet of paper and look through it with one eye, keeping the other open. Hold up the left hand in front of the other eye, close to the farther end of the roll, and you will seem to be looking through a hole in your hand.

12. Divide a white pasteboard disk into an even number of sections and blacken every other one, as shown in Fig. 2. Spin the disk rapidly by means of a Twirler and by looking at it Fig. 2. steadily it will Experiment 12. appear tinted,

the color changing with the speed of rotation. The disk generally appears greenish first, and then pinkish.

Another way of performing the experiment is to cut away sectors from a black disk and then rotate it between the eye and a cloudy sky. The sky will gradually assume different tints which vary with the speed of the disk. None of these colors are real, but caused by the excitement of the optic nerve by a rapid succession of darkness and light.

13. Cut in a piece of cardboard two square holes, each about half an inch square and a quarter of an inch apart. Procure a number of bits of glass of various colors, about an inch square, and fasten two behind the holes in the cardboard by means of elastic bands. Buy of an optician what is called a double-refracting prism, a piece of Iceland-spar or calc-spar which makes objects seen through it appear double. Hold the card up to a window or lamp and look through a prism at it. Each colored hole will appear double, and by holding the prism at the proper distance, one color can be made to overlap the other, so that the eye sees a mixture of the two. Note what this is. Now unfasten the bits of glass and look through both together at the light. The mixed color is entirely different from that obtained before. The reason is that in the first case one color really added its effect to the other, whereas in the second case the color seen is merely that remaining after each glass has strained certain colors out of the sunlight. Thus, suppose blue and yellow glass be tried. A mixture of pure blue and yellow light makes white, so the color seen through the prism will be whitish gray. But, when looked through together the glasses will appear green, because the rays of light are the only ones which will pass through both yellow and blue glass. In the same way red and green appear orange by the first method and dark green by the second; red and blue seem first violet and then deep red; and yellow and red appear first orange-yellow, then orange-red.

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14. Darken the room and admit a little daylight (not direct sunlight) through an opening. With this throw the shadow of a rod or other object on a white wall or screen, and light a candle, so as to throw a second shadow. Alter the size of the opening through which daylight is admitted, so as to make the two shadows as nearly as possible of the same intensity The shadow thrown by the candle is really white, since it is the only part of the wall on which pure daylight shines alone, yet by contrast it appears blue. If it be looked at through a roll of black cardboard or paper the part of the

wall about it will continue to appear blue, even when the candle is put out, but on removing the roll from the eye, it seems white again, and cannot be made to look blue except by lighting the candle a second time.

15. With a pair of compasses draw six or eight concentric circles, as near one another as possible. Make four dots, dividing the outermost circle into equal parts, and then join these dots by straight lines, drawn with the aid of a ruler. (Fig. 3.) These lines will appear to be curved inward. This is because they cross the circles at different angles.

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Experiment 15.

and the judgment of the observer cannot help attributing this, in part, to the curvature of the line.

16. Hold horizontally, a little below the eyes, a rod about a foot long, with its near end six or eight inches from the face and its opposite end pointing directly away. Look at the near end, and the two images of the rod will appear like a V, with the point toward the face. Fix the eyes on the farther end, and the V will have its point away from the face.

17. Press the closed eye with the finger tip close to the nose. A dark spot with a light border will be seen on the other side of the eye. If the eyeball he pressed on the outside the spot will be seen on the inside.

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Fig. 4.—Experiment 19.

who does not know, be asked which is the largest of the upright lines, he will be apt to point out the one crossed by the greatest number of the converging lines.

20. After reading for some time with one side toward a window, close the eyes alternately, and it will be seen that the paper of the book has a greenish tinge when seen by that eye alone which was next to the window. This effect is stronger if the light be very bright.

The reason is that the light, shining through the blood-vessels in the eyelid, tries that part of the eye that appreciates red, and so a white page appears to it slightly tinted with the complementary or opposite color, green.

21. Observe the letter S in a book, for instance the one just given. The bottom and top seem to be of about the same size. Turn the book up

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Fig. 5, and place them as there shown. The eye will usually judge, at first sight, that the lower is the longer, if the pieces be made of different colors, to distinguish them, and their places be changed, one will seem to have decreased and the other to have increased.

23. Make a pinhole in a card and hold it three or four inches before the eye. Hold a pin-head as close to the eye as possible and it will be seen, upside down, in the pinhole. This is because, though the pin is much too near the eye to form an image on the retina, the ray of light through the pinhole causes it to cast a shadow there. This shadow is upright, whereas the images of objects are inverted, so, as they appear right side up, the shadow appears upside down. If several pinholes be made instead of one, the pin-head will be seen in each one of them, because each ray of light throws a separate shadow of the pin-head on

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