It would occupy too much of our time, and it would involve the discussion of matters which are rather beyond the scope of elementary lectures, if we were to enter into a complete analysis of all the arguments proving that this electric radiation, which proceeds from an electric oscillator, is really a wave-motion. I may, however, mention one fact, which has been the outcome of an enormous amount of experimental research, and that is, that the velocity of this electric radiation through space is identical with that of light. It has already been mentioned that a ray of light flits through space at the rate of 1,000,000,000 feet, or nearly 186,500 miles a second. By suitable and very ingenious arrangements, physicists have been able to measure the velocity of electric radiation, and have found in every case that its velocity of propagation is precisely the same as a ray of light. Let us, then, summarize briefly what we have learnt. We find that when we set up an electrical oscillation in an open circuit consisting of two metallic rods placed in one straight line, we have proceeding from this circuit an electrical radiation which is capable of being propagated through space, which moves in straight lines, can be reflected and refracted, can exhibit the phenomena of interference, and moreover which is propagated with exactly the same velocity as light. Is it possible to resist the conclusion that this effect which we call electric radiation, and the similarly behaving physical agency which we call light, must both be affections of the same medium? It is hardly necessary to occupy time with experiments in showing that a ray of light can be reflected and refracted by mirrors and prisms, and converged or diverged by transparent lenses. These are simple optical facts, and if you are not familiar with them it will be easy for you to make their acquaintance by studying any simple book upon optics; but I should like to draw your attention to the fact that, in addition to rays of light and electric radiation, we are acquainted with another kind of radiation, which is also susceptible of being refracted, and that is commonly called _dark heat_.