star to be green. But now suppose that the star is hurrying towards us, it follows that the number of vibrations received in a second by the eye will undergo an increase. For the relative movement is the same as if the earth were rushing towards the star. In this case we advance, as it were, to meet the waves, and consequently receive them at less intervals than if we were to wait for their arrival.
Many illustrations can be given of the simple principle here involved. Suppose that a number of soldiers are walking past in single file, and that while the observer stands still twenty soldiers a minute pass him. But now let him walk in the opposite direction to the soldiers, then, if his speed be as great as theirs, he will pass forty soldiers a minute instead of twenty. If his speed were half that of the soldiers, then he would pass thirty a minute, so that in fact the speed with which the observer is moving could be determined if he counts the number of soldiers that he passes per minute, and makes a simple calculation. On the other hand, suppose that the observer walks in the same direction as the soldiers; if he maintains the same pace that they do, then it is plain that no soldiers at all will pass him while he walks. If he moves at half their rate, then ten soldiers will pass him each minute. From these considerations it will be sufficiently apparent that if the earth and the star are approaching each other, more waves of light per second will be received on the retina than if their positions are relatively stationary. But the interpretation which the brain will put on this accession to the number of waves per second is that the hue of the light is altered to some shade nearer the blue end of the spectrum. In fact, if we could conceive the velocity with
