We can, then, proceed to show that this electric radiation can be reflected, just like light or sound, by metal or other conducting surfaces, and that the law of reflection of the electric ray is the same as the law of reflection for rays of light or sound. If we place the radiator A with its mouth upwards, still preserving the receiver B in a horizontal position, it is possible to adjust the two very near to one another, but yet so that the radiation from the radiator does not affect the receiver. If I now hold a metal plate, P, at an angle of 45° above the mouth of the radiator, you will notice that the bell at once rings, thus showing that the electric radiation has been reflected into the receiver-box (see Fig. 76). Also we find that a very small deviation from the angle of 45° is sufficient to prevent the effect. Careful experiments in the laboratory show that the electric ray is reflected according to the optical law, viz. that the angle of reflection is equal to the angle of incidence. We find that any good conducting surface will, in this manner, affect the electric radiation. Thus I can reflect it from a sheet of tinfoil or even from my hand, and the fact that I can, so to speak, take hold of this electric radiation, and deflect it in different directions by the palm of my hand, produces in the mind a very strong conviction that we are dealing with something of a very real nature in experimenting with this electric radiation. It will be in your remembrance that, in the chapter in which we were dealing with waves in the air, I showed you a very interesting experiment illustrating the refraction of rays of sound by means of a carbonic acid prism, and I have now to bring before you an exactly analogous experiment performed with electric radiation. Here, for instance, is a prism made of paraffin wax, a substance which you have already seen is transparent to the electric ray. If we arrange the radiator- and receiver-boxes at an angle to one another, it is possible so to adjust them that the electric radiation projected from the radiator-box A just escapes the receiver-box B, and does not therefore cause the bell to ring (see Fig. 77). When this adjustment has been made we introduce the paraffin prism P into the path of the electric ray, and if the adjustments are properly made, we find that the electric ray is bent round or refracted, and that it then enters the receiver-box and causes the bell to ring. This experiment was first performed by Hertz with a very large pitch prism, but his apparatus was too cumbersome for lecture purposes, and the smaller and more compact arrangement you see before you is therefore preferable for present purposes. [Illustration: FIG. 77.—The refraction of an electric ray.]