No achievement of modern science is more remarkable than the measurement of the velocity with which stars are moving to or from us. This is effected by means of the spectroscope through a comparison of the position of the spectral lines produced by the absorption of any substance in the atmosphere of the star with the corresponding lines produced by the same substance on the earth. The principle on which the method depends may be illustrated by the analogous case of sound. It is a familiar fact that if we stand alongside a railway while a locomotive is passing us at full speed, and at the same time blowing a whistle, the pitch of the note which we hear from the whistle is higher as the engine is approaching than after it passes. The reason is that the pitch of a sound depends upon the number of sound beats per second. A B X * . . . . . . . * . . . . . . . Now, we may consider the waves which form light when they strike our apparatus as beats in the ethereal medium which follow each other with extraordinary rapidity, millions of millions in a second, moving forward with a definite velocity of more than 186,000 miles a second. Each spectral line produced by a chemical element shows that that element, when incandescent, beats the ether a certain number of times in a second. These beats are transmitted as waves. Since the velocity is the same whether the number of beats per second is less or greater, it follows that, if the body is in motion in the direction in which it emits the light, the beats will be closer together than if it is at rest; if moving away they will be further apart. The fundamental fact on which this result depends is that the velocity of the light-beat through the ether is independent of the motion of the body causing the beat. To show the result, let A be a luminous body at rest; let the seven dots to the right of A be the crests of seven waves or beats, the first of which, at the end of a certain time, has reached X. The wave-length will then be one seventh the distance A X. Now, suppose A in motion toward X with such speed that, when the first beat has reached X, A has reached the point B. Then the seven beats made by A while the first beat is traveling from A to X, and A traveling from A to B, will be crowded into the space B X, so that each wave will be one seventh shorter than before. In other words, the wave-lengths of the light emitted by any substance will be less or greater than their normal length, according to the motion of the substance in the direction in which its light is transmitted, or in the opposite direction.