So for a hundred years or more people have, generally speaking, accepted the idea that light consists of waves in a medium called The Ether. Heat also is brought to us from the sun and from any distant hot body by similar means, the difference between light waves and heat waves being simply in their wave length or the distance apart. The different colours of light, too, are to be accounted for by different wave lengths. You have of course seen how a magnet can act upon a piece of iron at a distance. You may, too, have tried the experiment of jerking a magnet past a piece of wire, thereby generating an electric current in the wire. Both those things need, for explanation, that we assume the existence of a something invisible and undetectable by our senses between the magnet and the iron and between the magnet and the wire, by which the action of one is conveyed to the other. So people imagined another Ether capable of acting like a link between the magnet and the iron and between the magnet and the wire. Now just about half a century ago a celebrated professor of Cambridge University brought all these facts about light, heat, magnetism and electricity together and by skilful reasoning showed that but one Ether sufficed to explain all these things. He showed how magnetic and electric forces acting together could produce waves like those of light and heat. And finally he demonstrated by figures that waves so formed would necessarily travel at the very speed at which light and heat are known to move. This is known as the electro-magnetic theory of light. And not content with showing the nature of things already known, Professor Clerk-Maxwell added a prophecy that there were other waves in existence of longer wave length, which no one then knew how to make or to detect if made. Following up this prophecy many investigators sought these waves, and the first to find them was Professor Hertz of Carlsruhe in Germany. Fortunately for his position in the minds of English people he died before the War, so that his name is not sullied by the stupidities of which German professors in more recent days have been guilty. On the contrary, his writings show him to have been a kindly, modest, genial soul, and particularly gratifying is his generous assertion in one of his books that had he not himself discovered these waves he is certain Sir Oliver Lodge would have done so. He seemed quite anxious to share the credit of his discovery with his "English colleague" as he called him.