Let's look again at the coils of Fig. 28 which we studied in the last letter. I have reproduced them here so you won't have to turn back. When electrons start from _a_ towards _b_ there is a momentary stream of electrons from _d_ towards _c_. If the electron stream through _ab_ were started in the opposite direction, that is from _b_ to _a_ the induced stream in the coil _cd_ would be from _c_ towards _d_. [Illustration: Fig 30] It all reminds me of two boys with a hedge or fence between them as in Fig. 30. One boy is after the other. Suppose you were being chased; you know what you'd do. If your pursuer started off with a rush towards one end of the hedge you'd "beat it" towards the other. But if he started slowly and cautiously you would start slowly too. You always go in the opposite direction, dodging back and forth along the paths which you are wearing in the grass on opposite sides of the hedge. If he starts to the right and then slows up and starts back, you will start to your right, slow up, and start back. Suppose he starts at the center of the hedge. First he dodges to the right, and then back through the center as far to the left, then back again and so on. You follow his every change. [Illustration: Fig 31] I am going to make a picture of what you two do. Let's start with the other fellow. He dodges or alternates back and forth. Some persons would say he "oscillates" back and forth in the same path. As he does so he induces you to move. I am on your side of the hedge with a moving-picture camera. My camera catches both of you. Fig. 31 shows the way the film would look if it caught only your heads. The white circle represents the tow-head on my side of the hedge and the black circle, young Brown who lives next door. Of course, the camera only catches you each time the shutter opens but it is easy to draw a complete picture of what takes place as time goes on. See Fig. 32. [Illustration: Fig 32] Now suppose you are an electron in coil _cd_ of Fig. 33 and "Brownie" is one in coil _ab_. Your motions are induced by his. What's true of you two is true of all the other electrons. I have separated the coils a little in this sketch so that you can think of a hedge between. I don't know how one electron can affect another on the opposite side of this hedge but it can. And I don't know anything really about the hedge, which is generally called "the ether." The hedge isn't air. The effect would be the same if the coils were in a vacuum. The "ether" is just a name for whatever is left in the space about us when we have taken out everything which we can see or feel--every molecule, every proton and every electron. [Illustration: Fig 33] Why and how electrons can affect one another when they are widely separated is one of the great mysteries of science. We don't know any more about it than about why there are electrons. Let's accept it as a fundamental fact which we can't as yet explain. [Illustration: Fig 34]