The principle of the method is very readily explained. Light comes to us from the heavenly bodies, as from other luminous bodies, in waves, which sweep through the ether of space at the rate of about 185,000 miles per second. The whole of that region over which astronomers have extended their survey, and doubtless a region many millions of millions of times more extended, may be compared to a wave-tossed sea, only that instead of a wave-tossed surface there is wave-tossed space. At every point, through every point, along every line, athwart every line, myriads of light-waves are at all times rushing with the inconceivable velocity just mentioned. It is from such waves that we have learned all we know about the universe outside our own earth. They bring to our shores news from other worlds, though the news is not always easy to decipher.
All the celestial bodies are in motion amid the multitudinous waves of space. Something can be learned respecting their motions by studying the waves. If a strong swimmer were stemming a series of long, rolling waves, their crests would pass him in more rapid succession than if he were at rest; if, on the other hand, he reversed his course, so that waves overtook instead of meeting him, their crests would pass in slower succession. One can easily conceive how, if he knew the exact rate at which the crests would pass him—so many exactly per minute—were he at rest, their slower or more rapid succession might indicate how fast he himself was moving, either from or toward them. If he were quite unconscious of his own motion, the effect would be simply that the distance from crest to crest would seem to be diminished in one case, lengthened in the other—that is, the waves narrowed or widened. Similarly with the aërial waves which produce sound. They are seemingly shortened when the source of sound is approaching, whether by its own motion or the hearer's, and lengthened when the source of sound is receding. In the former case the tone of the sound is made more acute, in the latter graver than it really is. This is strikingly illustrated when a swift train rushes past a station, the whistle sounding all the time, for there is a perceptible lowering of the whistle's note as the. engine passes a nearer on the platform. While the train ishim, he hears a note somewhat sharper than the true note of the whistle; after it has passed he hears a note somewhat flatter than the true note. Still more obvious, even to non-musical ears, is the corresponding change when two trains pass each other. In America, where a hideously-clanging engine-bell is used, the change is very remarkable, insomuch that a person unfamiliar with the arrangement actually adopted would suppose a different bell was rung the moment the engine passed the hearer.
Light traveling also in waves, it is obvious that a similar effect must be produced by approach or recession, if only the rate of motion is sufficiently rapid. The swimmer of my first illustration must have