ments of the stars towards us or from us, as well as the movements perpendicular to the line of vision, which had always been the subject of much investigation. It is just here that the spectroscope comes in to fill the vacant place in the equipment of the astronomer. It tells exactly what the older methods were unable to tell, and it does so with a certainty that suggests vast possibilities for the spectroscopic process in the future. The principle of the method is a beautiful illustration of the extent to which the different branches of physical science are interwoven. But the principle has been a familiar one to astronomers for many years. It is the facility and success attending its recent application that has now aroused so much interest. Once it became certain that the undulatory theory of light expressed a great truth of nature, a certain deduction from that truth became almost obvious. It was, however, by no means certain that the practical application of this deduction to astronomical research would be feasible. That it has proved to be so in any degree is somewhat of a surprise, while it now appears susceptible of developments to an extent that could hardly have been dreamed of.
The logic of the new method is simple enough. Our eyes are so constituted that when a certain number of ethereal vibrations per second are received by the nerves of the retina, the brain interprets the effect to mean that a ray of, let us say, red light has entered the eye. A certain larger number of vibrations per second is similarly understood by the brain to imply the presence of blue light on the retina. Each particular hue of the spectrum—the red, orange, yellow, green, blue, indigo, violet—is associated with a corresponding number of vibrations per
