FIRST EXPERIMENT WITH THE HELIOSTAT.

Choose a bright sunny day, and take the heliostat into a room having a window facing the south. Raise the sash and place the instrument in the window, and fasten it there so that it will be firm and steady. Before closing the window down upon it, move the larger mirror on its axis till it reflects a beam of light into the small mirror. Then turn the handle to the right or left, and a round, horizontal beam of light will enter the room. When this is done, close all the windows, so as to make the room as dark as possible. To do this, shawls or blankets or enameled cloth will be found useful inside the curtains and shutters. Then get a piece of cardboard, about 6 inches (15.2 centimetres) square, and lay a five-cent piece in the centre, and, with a knife, cut a hole in the card just the size of the coin. Then fasten this, with pins or tacks, over the opening in the heliostat.

We have now a slender beam of light in a dark room. Walk about and study it from different sides. See how straight this slender bar of light is; it bends to neither the left nor right, but extends across the room in an absolutely straight line. As the sun moves, turn the handle of the heliostat to keep the light in place.

Here is a picture of a dark room, in the window

of which is the heliostat. In the centre of the piece of cardboard is the small hole where the light enters the room. A boy is holding one end of a long piece of linen thread just at the bottom of the hole in the

card, and another boy has drawn the thread out straight and tight, so that it just touches the beam of light throughout its length.

Were you to try this experiment, you would see

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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 2 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 blocks as shown in the picture. When each card is 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 a 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