In connection with the refraction of rays of sound, it was pointed out that a curved surface has the power to diverge or converge rays of sound, and you will remember that we employed a sound-lens for converging the rays of sound diverging from a whistle, just as an ordinary burning-glass, or double convex lens, can be employed to bring the rays of sunlight to a focus. We shall now attempt a similar experiment with the electric ray. A block of paraffin is fashioned into the shape of a semi-cylinder, flat on one side and convex on the other, and this plano-convex paraffin lens has a convex surface having a radius of 6 inches. If I place the radiator A and receiver B about 4 feet apart, then by making a few little adjustments it is possible to so arrange matters that the radiation which proceeds from the radiator is not powerful enough at a distance of 4 feet to sensibly affect the coherer and make the bell ring (see Fig. 78). If, however, I adjust the paraffin lens =L= halfway between, I shall converge this electric radiation to a focus just about the place where the coherer is situated, and the consequence is that on making sparks between the balls of the radiator, we find that the bell attached to the receiver at once rings. [Illustration: FIG. 78.—Converging a beam of electric radiation.] We have, therefore, here brought to a focus, by means of a paraffin lens, the electrical radiation just in the same manner that an ordinary burning-glass focuses the rays of light and heat of the sun, and enables us to light with it some paper or a cigar. We have, therefore, indubitable proof in all these experiments that we have something proceeding from the radiator which is capable of being reflected or refracted just like the rays of sound or ripples on the surface of water; and, moreover, we find that this electric radiation passes through some substances but not through others. There is, therefore, a strong presumption that we are here dealing with something which is similar in nature to light, although it cannot affect the eye. In order that we may complete the proof we must show that this radiation is susceptible of interference. This proof may be partly obtained from the consideration of the following facts connected with the opacity or transparency of wire grating to the electric radiation:—