This will be quite easily understood from the accompanying diagrams. In each of these diagrams the set of waves marked _a_ are supposed to be moving from left to right, while those denoted by _b_ are reflected back and are moving from right to left. It will be noticed that each wavy line has a straight line drawn through it, dividing it into alternate crests and hollows, which line is known as the axis of the waves. Now notice that in Fig. 8 there are points marked x, where the _a_ waves are just as much above the axis as the _b_ waves are below it, and vice versa. Hence at those points the two sets of waves will neutralise each other. Now turn to the next figure, which, be it remembered, shows the same waves a moment later, when they have moved a little farther on in their respective journeys, and it will be seen that there, too, are places marked x where the two sets of waves neutralise each other. And the same with the third diagram. And finally observe that the places marked x are always the same in all the diagrams--that is to say, they are always the same distance from the line on the right-hand side, which denotes the reflector. It will be clear, too, that each node is half a wave-length from the next. Thus it can be shown that at every moment, and not merely at the three indicated in the diagrams, the two sets neutralise each other at the nodes, that the nodes are always in the same places and half a wave-length apart. [Illustration: FIGS. 8, 9 and 10.--These diagrams help us to see how the "wireless waves" are measured. The _a_ waves are supposed to be moving from left to right and the _b_ waves from right to left. At the points marked x they neutralise each other. It is then easy to discover those points and the distance apart of any two adjacent ones is half the "wave-length." _N.B._--In Fig. 10 the _b_ waves fall exactly on top of the _a_ waves.] Everywhere else, except at the nodes, there is action more or less energetic, but _there_ is perpetual calm. But how can we tell where the nodes are? When we recollect that they are points at which no wave-motion at all takes place it is easy to see that we shall at those points get no spark in our detector. So what Hertz did was to set his oscillator going so that it threw waves upon a reflecting surface and then move his detector to and fro in the neighbourhood until he found the nodes. Between the nodes, as will be seen by an inspection of the curves once more, there are other points at which the wave-action will be twice as great as with the single wave, and so at those points the response of the detector would be especially energetic. This mutual action between an incident wave and a reflected wave is termed "interference," and by it the wave-lengths of all the ethereal waves have been measured. The plan used in the case of light waves, although the same in principle, is somewhat different because of the extreme shortness of the waves.