Page:In the high heavens.djvu/34

 distance from the various points around. That point on the celestial sphere where the star shows the velocity of approach to be greatest, can only be the point towards which the solar system is directing its motion. The opposite point of the heavens is that from which the movement of retreat is greatest. At intermediate points there will, throughout one hemisphere, be indications of approach, and throughout the other indications of retreat.

The stars employed in these researches are those which have been investigated spectroscopically by Professor Vogel in his memorable Potsdam observations. The number of the stars made use of was 51. It must be observed that though the method under consideration is admirably adapted to discover the speed of the solar system, it is not equally fitted to indicate the position of the point on the heavens towards which the sun's motion is directed. It can be shown that the conditions are such as to render the detection of that position by the use of the spectroscope much inferior to the determinations afforded by the former process employed by Stumpe. I therefore only use the spectroscopic method for the determination of the velocity deduced on the supposition that the apex of the sun's way is indistinguishable from the position of Delta Lyræ. The result of Vogel's investigation is to show that the velocity of the solar system is about eight miles a second. The accuracy possessed by this result is as usual best indicated by its probable error. It can be shown that the probable error of the statement, that the solar velocity is eight miles per second, is about two, that is to say, it is equally probable that the error of the result lies below two miles a second as that it lies above.