The great revelations of our times have come through the application of the spectroscope to the measurement of motions in the line of sight from us to a star. No achievement of the intellect of man would have seemed farther without the range of possibility to the thinker of half a century ago, than the discoveries of invisible bodies which are now being made with this instrument. The revelations of the telescope take us by surprise. But, if we consider what the thinker alluded to might regard as attainable, they are far surpassed by those of the spectroscope. The dark bodies, planets, we may call them, which are revolving round the stars, must be forever invisible in any telescope that it would be possible to construct. They would remain invisible if the power of the instrument were increased ten thousand times. And yet, if there are inhabitants on these planets, our astronomers could tell them more of the motions of the world on which they live than the human race knew of the motions of the earth before the time of Copernicus. The men and institutions which have contributed to this result are so few in number that it will not be tedious to mention at least the principal actors. The possibility of measuring the motions of the stars in the line of sight by means of the spectroscope was first pointed out by Mr. now Sir William Huggins. He actually put the method into operation. As soon as its feasibility was demonstrated it was taken up at Greenwich. In these earlier attempts, eye methods alone were used, and the results were not always reliable. Then spectrum photography was applied at the astrophysical observatory at Potsdam by Vogel. Thence the photographic method soon spread to Meudon and Pulkova. But, as often happens when new fields of research are opened, we find them ablaze in quarters where we should least expect. The successful application of the method requires not only the best spectroscope, but the most powerful telescope at command. Ten years ago the most powerful telescope in the world was at the Lick Observatory. Mr. D. O. Mills put at its eye end the best spectrograph that human art could make at that time, the work of Brashear. It is Campbell, who, with this instrument, has inaugurated a series of discoveries in the line in question which are without a parallel. A mere survey of what has been done in the various lines we have mentioned would be far from giving an idea of the real significance of the advance we are considering. Cataloguing the stars, estimating their magnitudes, recording and comparing their spectra and determining their motions, might be considered as, after all, barren of results of the highest human interest. When we know the exact position of every star in the heavens, the direction in which it is moving and the character of its spectral lines, how much wiser are we?