between the others, and in a moment you will see the yellow star shining through all three cards.

Next take a piece of thread and stretch it against the sides of the three cards, just as they stand, and immediately you see that they are exactly in line. The holes in the cards we know are at the same distance from the edges of the cards, and our experiment proves that the beam of light that passed through all the holes must be straight, or we could not have seen it. The cards are in a straight line, and the beam of light must also be straight. This experiment, like the first, shows us that there is a law or rule governing the movement of light, and that law is, that light moves in straight lines.

Move the lamp as near to the edge of the table as possible, and then bring one of the cards close to the lamp-chimney. Then change your seat, and repeat this experiment several times in different directions. Each time you will see exactly the same thing, no matter in what direction the light moves from the lamp. The lamp may be moved from one side of the table to the other, and in every direction we shall find the light moving in exactly straight lines from the source of light. This is true whether the source be the sun, a lamp, or a star. One can walk all about the lamp and see it from every side, and we can place

our three cards in any direction, north or south, up or down, east or west, or in any and every direction, and every time it will give the same result.

Thus we have found out the law by which light moves, viz., it moves in straight lines in all directions. from the source of light.

A

Knowing this, you can readily think of a number of things in which these laws are made useful. farmer planting an orchard, an astronomer fixing the positions of stars, a sailor steering his ship by night, employs this law: the first, to arrange his trees in straight lines; the second, to measure out vast angles in the sky; and the third, to lay the courses of his ship in safety. Each employs these laws with certainty and safety, because they are fixed and never change.

EXPERIMENT WITH SHADOWS.

This picture represents a sheet of white note-paper, standing upright, like a small screen, upon a table. Near it is a bit of square paper, fastened to the end of our needle-pointed awl, and beside this is a lamp, and next to the lamp is a postal-card, having a slit cut in it near the top. On the screen you will notice that there is a shadow of the bit of paper held on the needle. The paper screen may be made of

any sheet of white paper, and it may be held upright by placing some books behind it. The bit of paper on the needle is just 1 inch (25 millimetres) square; and to hold the awl in place, the handle

FIG. 5.

may be stuck in a mass of wax on the table. The slit in the postal-card should be 1 inch (25 millimetres) long and inch (7 millimetres) wide, and should be horizontal. The card may rest against the lamp, and, if it is not high enough, put something under it, so that the slit will be opposite the flame. These things are casily procured, and, when you have them, light the lamp, place the postal-card before it, and then make the room quite dark, or, if it is night, put out all the other lights. Set up the needle-awl with the bit of paper on the end about 12 inches (30.5 centimetres) from the lamp, and make it firm and

steady with a bit of wax softened in the fingers. Then bring the screen in a line with the paper square and the lamp, and about 24 inches (61 centimetres) from the lamp. If everything is right, there will be a square shadow of the bit of paper on the screen. Look carefully at everything, and have the paper just on a level with the slit in the postal-card, and have the lamp, paper, and screen, just in line, and then the square shadow will appear sharp and clear on the white screen. With a lead-pencil trace an outline of this shadow on the screen; then move the screen just 12 inches (30.5 centimetres) farther from the lamp. Look at the shadow. See how it has increased in size. With the pencil trace this shadow on the screen, and then, laying the screen on the table, measure the two shadows, and see how they compare in size, and see how they both compare with the size of the paper square that cast the shadows on the

screen.

Fig. 6 shows how light spreads out, and how shadows expand as the distance increases. A is the lamp, and B is the postal-card, having a slit for the light to pass. C is the paper screen, and D is the first shadow made on the screen when it was 24 inches from the lamp. E is the second shadow made on the screen when it was 36 inches from the lamp.

If you lay the paper C on the tracing of the small shadow D, you will observe that it only covers onefourth of the surface, and that the shadow is four times as large. Place it on the larger shadow E, and you will see that it covers only one-ninth of its surface. In the diagram the first shadow is marked off into quarters, and the second into ninths, by dotted lines. The diagram also shows how the rays of light spread out wider and wider the farther they travel from the source of light.

Now, make two squares of paper, one the size of D and the other the size of E. Then place D

24 inches (61 centimetres) from the lamp, and E 36 inches (91.4 centimetres), and both in a line. If C, D, and E, have the positions shown in the diagram, it will be found that D and E are both in shadow, while the square C is illuminated. Remove the square C, and D will be lighted, which shows