that the thread would suddenly be lighted up throughout its whole length, and would shine in the dark room like silver. Then if the boy allows the thread to become slack and loose, or if he lowers it even a very little, it will disappear in the darkness. If he raises and lowers it quickly, it will seem to appear and disappear as if by magic.

This is a very pretty experiment; but we must not stop to look at its merely curious effects. Try it over several times, and see if it does not show you something about the beam of sunlight. Plainly, if the thread is lighted up its whole length when it is straight, then the beam of light must be straight also. Here we discover something about light; we learn that it has a certain property. Our experiment shows that light moves in straight lines.

EXPERIMENT WITH CARDS AND A LAMP.

Here is a picture representing three little wooden blocks placed in a row upon a flat, smooth table, and fastened to them are three postal-cards, so that they will stand upright. At the end of the table is a small lamp. This is all we need to perform another experiment, that will show us the the same thing we observed with the beam of light from the heliostat. To make these things, get a piece of

wood 10 inches (25.4 centimetres) long, 3 inches (76 millimetres) wide, and 1 inch (37 millimetres) thick, and saw it into five pieces, each 24 inches

(64 millimetres) long.

Next make three slips of

pine, 4 inches (10 centimetres) long, 3 inches (76

millimetres) wide, and

inch (4 millimetres) thick. Having made these, get three postal-cards, and lay them flat on a board, one over the other. Just here we need a tool for making small holes and doing other work in these experiments; and we push, with a pair of pliers, a cambric needle into the end of a wooden penholder, or other slender stick, putting the eye-end into the wood, and thus making a needlepointed awl. Measure off one-half inch from one end

of the top postal-card, and with the awl punch a hole through them all, just half-way from each side. Lift the cards up, and with a sharp penknife pare off the rough edges of the holes, and then run the needle through each, so as to make the holes clean and even.

Take one of these cards and one of the wooden slips, and put the card squarely on one of the wooden blocks and place the slip over it, and tack them both down to the block. This will give us the cards and the picture. When each card is

blocks as shown in

thus fastened to a block, we shall have two blocks left. These we can lay aside, as we shall need them in another experiment.

Now light the lamp, and place one block on the table, quite near the lamp. Look at the lamp carefully, and see that the flame is just on a level with the hole in the card. If it is too high or too low, place some books under it, or put the lamp on a pile of books on chair near the table. Take a chair and sit at the opposite end of the table, and place another card before you. Now look, through the hole in this card, at the first card before the lamp. If the table is level, you will see a tiny star or point of light shining through the holes in the two cards. Without moving the eye, draw the third card into line

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.

Knowing this, you can readily think of a number of things in which these laws are made useful. A 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