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If you look at the diagram you will see that the mirror is at an angle of 45 degrees with the beam of incidence, and that the beam of reflection is at an angle of 90 degrees with the incident beam. Hence, if the mirror is tilted through a certain angle, the reflected beam is tilted through twice this angle. For instance, if the mirror is moved 1 degree the

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beam of reflection moves 2 degrees. Place the mirror at an angle of 22 with the beam of incidence, and the beam of reflection is at an angle of 45. Move the mirror to an angle of 674, and the beam of reflection will move round to an angle of 135 degrees.

This drawing represents the two postal-cards fitted

on blocks of wood that we used in a former experiment, and the three blocks of wood we cut out at that time. The five blocks are placed close together in a line, and with the postal-cards at the ends. A lighted lamp is placed near one of the cards, and on the middle block is a small piece of window-glass that has been painted with black varnish. A single coat of black varnish on one side of the glass is all that is required to give us the black mirror needed in this experiment. Place the lamp close to the card in such a position that the flame will be just on a level with the hole in the card. If the lamp is not convenient, the blocks and cards may be placed upon a table facing a north window in full daylight.

When everything is ready, look through the hole in the postal-card marked B, down upon the black mirror, and on it you will see a single spot of light, the reflection from the lamplight or the light from the window shining through the hole marked A in the drawing. Get the needle-pointed awl and place it so that the point will just touch the spot of light in the black mirror, and then fasten the awl in this position with a piece of wax, as represented in the picture.

You will readily see that this experiment is the same as the last. Again we have a beam of light

reflected from a mirror. The beam of incidence passes through the postal-card at A and finds its point of incidence on the mirror, and the beam of reflection extends from the point of incidence to the second card at B.

Take a sheet of stiff paper 10 inches (25.4 centimetres) long, and about 4 inches (10 centimetres) wide, and hold it upright between the two cards, with the bottom resting on the mirror. With a pencil make a mark on the edge of this at the point of incidence, marked by the awl, and at the hole in the card where the beam of incidence enters, and marked A in the drawing. Draw a line between these two points and you have an angle formed by this line and the base of the paper. This angle marks the angle of incidence. Put the paper on the blocks, with the ruled line toward the card B, and you will find that the line fits here equally well. It now extends from the point of incidence to B, and proves that this angle is the same as the other, that both sides are alike, and that the angle of incidence and the angle of reflection are equal.

Take out the block in the middle, and move the others nearer together till they touch. Repeat the experiment: make a measurement with a piece of paper as before, and draw a line on it from the point

of incidence to either of the holes on the cards, and then compare the angles thus found, and in each case they will be exactly alike. Take out another block and try it again, and you will reach the same result.

These experiments show us that there is a fixed law in this matter, and the more we study it the more we are convinced that it has no exceptions.

EXPERIMENT IN MULTIPLE REFLECTION.

Choose a south room on a sunny day, and close the blinds and shutters at all the windows save one, and at this window draw down the curtain until only a narrow space is left at the bottom. Close this space with a strip of thick wrapping-paper, and then cover the rest of the window with a blanket or shawl, so as to make the room perfectly dark. Then cut a round hole, the size of a five-cent piece, in this paper, and through this hole a slender beam of sunlight will fall into the darkened room.

Bring a hand-mirror into this beam of light, and the beam of reflection will make a round spot of sunlight on the wall above the window. This spot of light is a picture of the sun thrown by the mirror upon the wall. Hold the mirror at an oblique angle in the sunbeam, and direct the beam of reflection upon the opposite wall. Now there are several re

flections, brilliant spots of light. If the spots of light do not stand out sharp and clear, turn the mirror slowly round and you will soon find a position for the glass that will give six or more reflections.

How does it happen that a common looking-glass can thus split a single sunbeam into several beams? If you touch a pencil to a mirror you will notice that while the point of the pencil touches the glass the point of the reflected pencil seen in the mirror does not meet the point of the real pencil, and that there is a little space between them. The reflection we see in the glass is from the smooth surface of the quicksilver at the back of the glass, and the space between the reflection and the pencil is filled by the glass.

Hold a sheet of common window-glass before a lighted lamp or candle, and you will see a faint reflection of the flame in the glass, and at the same time you can readily see through the glass. shows us that the outside of any piece of smooth glass will reflect light, and our experiment is designed to show a still more curious matter.

This

Fig. 10 represents the single beam reaching the point of incidence on the outside of the mirror at O, and reflected to the wall at 1. Part of the light goes through the glass to B, and here is another point of incidence, and a new beam of reflection is thrown

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