cards flat on a board, one over the other; measure off a 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. Trim the holes with a pen-knife, and then take one of the cards and one of the wooden slips and put the card squarely on one of the wooden blocks, and, placing the slip over it, tack them both down to the block. Place one of the blocks near a lighted lamp, as shown in the figure, and another at the
Fig. 1.—Experiment proving that Light moves in Straight Lines.
opposite side of the table, where the observer can sit to look through the aperture. When the light is seen through both openings, draw the third card into line between the others, when the ray will be seen to pass through all three cards. Next, take a piece of thread and stretch it against the sides of the three cards as they stand, and it will be seen that they are exactly in line, and, as the holes in the cards are at the same distance from their edges, it is proved that the beam of light that passes through all the holes must also be straight. If the position of the blocks is changed, so that the directions of the holes in the cards are different, the same effect will be observed, so that it is demonstrated that light moves in exactly straight lines in all directions from the source of illumination. Of course, a pupil can learn from a book that light moves in straight lines, but this will be a matter of hearsay or authority, and not of direct knowledge, while if he makes this experiment he will have begun to prove things for himself, and the preparation for it, and trial in different ways, will be a good exercise in manipulation.
Now, if the student wishes to prove the variation in the quantity or intensity of light at varying distances, he can do it in the simple way shown in Fig. 2. A small slit is cut in the card near the lamp, through which the light passes. A sheet of white paper, resting against some books at the opposite side of the table, forms a screen, upon which the light falls. A bit of paper, an inch square, is held by the point of the awl, the handle of which is stuck in some
