Thus a little train of oscillations is produced for every impulse from the coil. Every train causes a corresponding disturbance in the ether, and sends off a train of electro-magnetic waves, and these, falling upon the distant wire, generate in it a train similar to that which brought them into being. These trains, in Hertz' simple apparatus, manifested themselves in the form of minute sparks leaping across the small gap between the ends of the curved wire (Fig. 7). [Illustration: FIG. 7.--Hertz "Detector." It was with this simple apparatus that Hertz discovered how to detect the "wireless waves."] It was in 1888 that Hertz made this discovery of a way to detect long electric waves. He subjected the matter to many more experiments and found that the waves have many points in common with light rays. He found that they were reflected from certain surfaces, just as light is reflected from the surface of a mirror. He made prisms which were able to bend them as light waves are bent by a prism of glass. Some things appeared to be transparent to them, as clear glass is to light, while others are opaque. It does not follow that the same things which reflect light waves reflect electric waves, and so on. The latter can pass through a brick wall, for example. But the same divergence is to be observed between light and radiant heat, of which the action of glass is a familiar example. Clear glass will let light through almost undimmed, yet we use it for fire-screens to shield us from too much radiant heat. The important fact is that all three--light, radiant heat and Hertzian waves--in addition to travelling at the same speed, are reflected, absorbed or refracted, according to precisely the same principles. This is almost perfect testimony to their essential identity. The difference between them, as has been said already, is the distance from crest to crest of the waves--the "wave-length," that is. And the reader will wonder by what manner of means this mysterious dimension can be ascertained. In spite of its seeming mystery the method is very simple. It is based upon the fact that two sets of similar waves travelling at the same speed in opposite directions interfere with one another in a peculiar way. Suppose that one set of waves travel along to a reflector and strike it vertically; then another set will travel back from the reflector exactly similar to the first, except that their direction will be opposite. And the result will be that at certain intervals they will exactly neutralise each other, so that at those points there will be no wave-action appreciable at all. Those points where no action is to be perceived are called "nodes," and they are exactly half a wave-length apart.