interesting experiment in measuring light, and we have only to make the room dark, or put out all the other lights, if it is evening, and we can go on with the work. Let the candle burn a moment or two, and then bend the wick down, so as to give a large flame. If you have no lamp, a gaslight will answer. Upon the paper screen are two shadows of the awl side by side. Move the lamp to the right or left till the two shadows just touch, and make one broad band. Study this double shadow carefully. Perhaps one half is darker than the other. Move the lamp backward or forward, and you will see that its shadow changes-bécomes darker or lighter. Presently you will find a place for the lamp where the double shadow appears of a uniform depth.

Now both lamp and candle cast just as deep a shadow, and yet one is much farther from the screen than the other. Measure off the distance. Perhaps the candle is 22 inches (55.8 centimetres) from the the screen, and the lamp is 44 inches (112 centimetres).

In our last experiment we found that the illumination of a given surface varies inversely as the square of its distance from the source of light. The square of 22 is 484, and the square of 44 is 1,938. Now, if we divide 1,938 by 484, we get 4, and thus

we find that our lamp is four times as bright as the candle. It casts just the same depth of shadow on the screen as the candle, and it is four times as bright, because the square of the distance of the candle will divide the square of the distance of the lamp four times. If we measure it another way we find the candle is half the distance from the lamp to the screen, and gives only one-quarter as much light.

Such a measurement as this is both easy and simple, and by means of such an experiment we can find out how much light any lamp gives. In this case, we find one lamp gives just four times as much light as the candle, or as much light as four candles would give at once. This is called a photometric experiment, from two Greek words meaning lightmeasurement. You may sometimes hear people say that a certain gas-lamp gives a sixteen or eighteen candle light, and our experiment shows us what they mean by this expression. They mean that the lamp has a photometric value of so many candles, or gives a light equal to the light of sixteen or eighteen candles burning at the same time.

CHAPTER III.

REFLECTION OF LIGHT.

PLACE the heliostat in position, and bring a slender beam of light into the darkened room. Then get a small looking-glass, or hand-mirror, and a carpenter's steel square, or a sheet of stiff paper, having perfectly square corners. Hold the mirror in the beam of light. At once you see there are two beams of sunlight, one from the heliostat and another from the mirror. Hold the glass toward the heliostat, and you will see this second beam going back toward the window.

This is certainly a curious matter. Our beam of light enters the room, strikes the mirror, and then we appear to have another. It is the same beam, thrown back from the glass. This turning back of a beam of light we call the reflection of light.

Place a table opposite the heliostat, and place the mirror upon it, against some books. Turn the mirror to the right, and the second or reflected beam of light moves round to the right. Turn the glass still more, turn off at a right angle,

and the beam of light will

and there will be a spot of light on the wall at that

side of the room. Now bring the carpenter's square or the piece of square paper close to the mirror, so that the point or corner will touch the glass just where the sunlight falls upon it. Now one edge of

the square is brightly lighted by the sunbeam, and if the mirror is placed at an angle of 45 degrees with the sunbeam, the other edge of the square is lighted up by the second beam.

In this diagram, A is the beam of light from the

heliostat, and B is the beam reflected from the mirror, that is marked M. To make this more simple, we call the first beam the beam of incidence, and we say that it travels in the direction of incidence, as shown by the arrow. The second beam, marked B, we call the beam of reflection, and the course it takes we call the direction of reflection. The point marked 0, where the light strikes the mirror, is called the point of incidence.

In the diagram is a dotted line representing a quarter of a circle reaching from the beam of incidence to the beam of reflection. A quarter of a circle, as you know, is divided into 90 degrees. Another dotted line extends from O at the mirror to I on the quarter-circle, and divides it into two parts. Half of 90 is 45, and hence the mirror stands at an angle of 45 degrees with both beams of light. Now the line A and the dotted line reaching from 0 to X make the angle of incidence, and the angle between B and the line from 0 to X is the angle of reflection; and the curious part of this matter is, that these two angles are always equal. Here they are both angles of 45 degrees.

Move the mirror about in any direction, and measure the angles of incidence and the angles of reflection, and these angles will always be exactly equal.