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

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 easily 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

that all the light that falls on D previously fell on C. E yet remains in darkness. Next, remove D and replace C, and E still remains in shade; but, on removing C, E is fully illuminated. This shows that the quantity of light that fell on C spreads over four times the surface at the distance D, and nine times the surface at the distance E. Hence each one of the squares on D is one-fourth as bright as the square C, and any one of the squares on E is one-ninth as bright as C.

Here we are coming upon another fact about light; we find another law governing its action. At one foot from the lamp the light had a certain power; at two feet it had only one-quarter as much power; at three feet it only had one-ninth as much power or intensity. So, if we approach the lamp, at a certain distance the light has a certain brightness; at half that distance it has four times the brightness; at one-third the distance it has nine times as much brightness. The above relation, existing between the intensity of the light on a surface at different distances from the source of light, is often stated as follows: The illumination of a given surface varies in brightness inversely as the square of its distance from the source of light.

EXPERIMENT IN MEASURING LIGHT.

This picture represents a sheet of white paper, standing upright upon a table. A few inches from this screen is our needle-pointed awl, stuck upright in the table. (If you do not care to do this, the awl can be stuck into a block of wood or bit of wax.) A lighted candle is placed on the table, about 22 inches (55.8 centimetres) from the screen. Beyond this is a lamp, placed upon a pile of books, so as to bring the

flame of the lamp on a level with the flame of the candle. The lamp should stand, say, at 44 inches (112 centimetres) from the screen; and, if it has a flat wick, it must be so placed that the wick stands diagonally to the screen.

These are all the things we need to make a most