to enter the eye just as sound may be said to enter the ear. We have already explained that when we hear the report of a gun, it does not mean that small particles of air travel all the way from the gun to our ear. And so when we view a ray of light it does not mean that a small particle is shot from the bright body into our eye. An impulse or wave in each case passes over the medium between us and the body, and the blow goes from particle to particle after the manner we have already explained in the experiment with ivory balls (Art. 44).

69. Reflection of Light.-When light strikes a polished surface of metal, it is reflected from it. If you

hold a lighted candle before a mirror, you will see the image of the candle in the mirror, which means that the rays from the candle strike the mirror and are reflected from it to your eye, just as if they came from the mirror itself and not from the candle.

* EXPERIMENT 49.-In order to understand how reflection acts, let us take a horizontal polished metallic surface-that is to say, let us pour mercury into a shallow flat-bottomed vessel. Now place a bent tube open at the bottom above the mercury as in fig. 29, and let the light of a candle enter the tube at the right end if we place our eye at the left end, we shall

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see the light from the candle as it comes reflected from the surface of mercury.

In this experiment, therefore, the light of the candle goes down the one tube, strikes the surface of mercury and then ascends the other tube to the eye. But in order that the light may do this, two things are necessary. In the first place, the two tubes must have the same inclination or slope; and secondly, the one tube must be exactly opposite the other, so that if they were suddenly to fall flat down they would be in a line with one another. Whenever, therefore, a ray of light strikes a polished surface, the reflected ray rises from the surface with the same slope as the ray that strikes the surface falls towards it, and both rays, if you could imagine them squeezed flat against the surface, would be found to make one line.

You cannot understand the laws of reflection com pletely without geometry, but the following figure will perhaps enable you to do so to some extent. In the figure, a is supposed to be a bright point giving out light, and M M is a mirror. Let A B, A B' be two of the rays of light from A, striking the mirror at в and B'. These will then rise into the eye of the observer in the directions BD, B'D', the falling slope of the ray A B being equal to the rising slope of B D, and the falling slope of A B' equal to the rising slope of B'D'. Now if you imagine the direction of the two rays B D, B'D', prolonged beneath the mirror, they would meet at a', a point as much below the mirror as the bright point A is above it. To the eye, therefore, the rays will in point of fact appear to proceed from A', so that the apparent position of the reflected image a' is as much behind the mirror as the bright point A is itself before it.

Whenever, therefore, you stand in front of a mirror, you see your own image in the mirror as much behind the mirror on the other side as you yourself are in front of it; if you go close to the mirror, the reflected figure goes close also, if you draw back the reflected figure draws back, and so on. You will, however, notice the difference-namely, that your right hand is the left hand of the image, and your right

side generally the left side of the image, but in other respects the image is precisely a copy of yourself.

In fig. 31 you see in the lower part the image of the upper part, and you notice how, in the image, the letters go from right to left, and not from left to right.

When the bright reflecting surface is not flat, curious images are sometimes produced. Take, for instance, the bright surface of mercury in the bulb of the thermometer and look into it. You will there see a very small

distorted image of yourself, and indeed of the whole room, only the far away parts of the room will be exceedingly small.

Take again a couple of bright concave mirrors like those of fig. 22, only, instead of putting a watch at the focus of the one mirror, and your ear at that of the other, place a red-hot ball in the one focus, and your hand in the other focus, and you will soon find it too hot. Indeed, if you had two large reflectors of this kind

and had a fire burning in the focus of the one, you might cook a beef-steak in that of the other, even though the two reflectors were fifty feet apart. The reason is that the rays of heat from the fire in the one focus strike the mirror near it, and are reflected from it in lines that bring them to the other reflector, and they are then again reflected in such lines as to bring them all together into the focus of this reflector. We thus have,

as it were, the fire itself burning at the one focus, and an image of the fire at the other, the image being sufficiently hot to cook a beef-steak.

70. Bending or refraction of Light. EXPERIMENT 50.-Put a small, heavy body at the bottom of a stoneware or pewter jug, and put your eye in such a position that the side of the jug just hides the body from your eye; then let some one fill the jug full of water, and the small body at the bottom will now become visible. Why is this? It is because the ray of light from the

small object at the bottom of the water after it leaves the surface of the water is bent in a different direction, so that you can in fact see it round a corner; and if the small body at the bottom were a little fish, it could also see you.

It thus appears that if a slanting ray of light strikes a surface of water, it is bent so as to be less slanting after it enters the water; or again, if a ray of light comes out from the water, it is bent so as to be more slanting after it enters the air. The same thing would happen if the ray of light entered a surface of transparent glass