This is not the only case in which we alter the positions of definite lines in the spectrum. Indeed, we have given man some interesting information about the motions of distant stars--information which he could not have obtained in any other way. We have sent wireless messages from distant stars, indicating that they were approaching the earth, while electrons aboard other stars have signalled that they are receding from the earth. All this may seem mysterious to you, and yet our actions in the matter are very simple. Indeed, we do nothing but what I have told you of in the preceding chapters. We send out definite wave-lengths in the manner described already. But if we are on board a star which is travelling towards the earth, our waves will naturally follow a little closer at each other's heels. On the other hand, if the star is receding from the earth, the waves must be a little farther apart than they would be if the star were at rest. You will understand that the electrons are revolving at the same speeds in both cases, but the forward movement of the star crowds the waves together, while a receding star stretches them out a little farther apart. The result at the receiving end is that the crowded waves are just as though they had come from electrons revolving at a greater speed than is actually the case. Hence the line appears farther along the spectrum, up the scale of frequencies, than would have been the case had the star not been moving forward in the line of sight. Thus if the hydrogen lines, of which I have spoken elsewhere, should appear higher up the spectrum than usual, then man knows that the star from which these waves are coming is approaching the earth. It will be evident that when known lines in the spectrum are shifted down the scale (towards the red end of the spectrum), then the rate of the waves has been decreased, and man knows that the star carrying these stimulating electrons is receding from him. You will observe that we electrons perform no new duty in connection with this matter; it is entirely the motion of the body carrying us that alters the positions of the lines. But I must hasten on to tell you of some personal experiences. CHAPTER XVII MY X-RAY EXPERIENCES _THE SCRIBE'S NOTE ON CHAPTER SEVENTEEN_ The present generation were all very much interested in the discovery of X-rays. With the aid of a battery and an induction coil, man causes an energetic electrical discharge to pass through a vacuum tube. When the flying electrons strike upon a little metal target placed in their path, they produce the well-known Roentgen rays. We have all become familiar with the great penetrating powers of these rays. The electron may be left to tell its own story. CHAPTER XVII MY X-RAY EXPERIENCES