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.