Supposing that we take an iron ball and make it red hot in a furnace, then, in a perfectly dark room, we see the ball glowing brilliantly, and we are conscious by our sensations that it is throwing off heat. Let us imagine that the ball is allowed to cool down to a temperature of about 500° C.; it will then just cease to be visible in a perfectly dark room, but yet if we hold our hand or a thermometer near to it, we can detect its presence by the dark radiant heat it sends out. Experiments show that even when the ball is brilliantly incandescent, nearly 98 or 99 per cent. of all the radiation it sends out is dark heat, and only 1 or 2 per cent. is radiation which can affect the eye as light. It is quite easy to show that this dark heat can be reflected just like light. If I fix this red-hot ball in the focus of a metallic mirror and lift up ball and mirror nearly to the ceiling and then place upon the table another convex, polished, metallic mirror, the top mirror will gather up and project downwards the radiation from the iron ball and the bottom mirror will converge that to a focus. If then we fix a red-hot ball in the focus of the upper mirror and allow it to cool until it is just not visible in the dark, we shall find that we can still ignite a piece of phosphorus or some other inflammable substance by holding it in the focus of the bottom mirror, thus showing that the dark radiation from the iron ball is susceptible of reflection just as are rays of light or electric rays. In fact, if time permitted, it would be possible to show a whole series of experiments with dark radiant heat which would prove that this radiation possesses similar properties of luminous or electric radiation in its behaviour as regards reflection, refraction, and interference. A vast body of proof has been accumulated that all these forms of radiation are merely varieties of one and the same thing, and that the only thing in which they really differ from one another is in what is called their wave-length. At this point I will remind you once more of that general law which connects together the velocity of propagation of a wave-motion, the wave-length and the frequency. It is expressed in the formula: _wave-velocity_ (V) _equals frequency_ (_n_) _multiplied by wave-length_ (λ), or in symbolical language— V = _n_λ Accordingly, if the velocity of propagation can be determined, and if the frequency or periodicity of the wave-motion is known, then the wave-length can be found from the above simple rule; or conversely, if the velocity of propagation and the wave-length are known, the frequency is determined.