Page:In the high heavens.djvu/165

 hands on a suitable star is shown by observations on three nights of the star Arcturus, the largest divergence of Keeler's measures being not greater than six-tenths of a mile per second, while the mean of three nights' work agreed with the mean of five photographic determinations of the same star at Potsdam to within one-tenth of an English mile. These are determinations of the motions of a sun so stupendously remote that even the method of parallax practically fails to fathom the depth of intervening space, and by means of light waves, which have been, according to Elkin's nominal parallax, nearly 200 years upon their journey."

It is impossible for any lover of astronomy to read of these achievements without some emotion. The alliance between photography and spectroscopy is here rendered available for extending our knowledge of the movements of the heavenly bodies in a direction wholly inaccessible to every other appliance of the astronomer. I may mention one of the points in which the importance of the new method can hardly be overrated. In the older process of ascertaining the proper motions of stars, the lapse of long periods of time was indispensable. A star would have to possess a movement more rapid than that of any of the stars, except a very few, if it could be determined by our meridian instruments in less than a twelvemonth. In the majority of cases an interval of many years would be necessary before the movement of the star could be certainly concluded from such measurements. With such small movements as those possessed by most of the stars, various causes combine to render the measurements highly uncertain; and yet for astronomers who desire to learn the constitution of the heavens, there would be no