Page:Popular Science Monthly Volume 37.djvu/518

502 measure, and draws fine lines of connection between earth and sky. One of these lines was drawn some thirty years ago in spectrum analysis; and by its aid man has risen in mind to remote worlds, and has sounded their physical and chemical constitution. The same spectrum analysis has now again celebrated a great triumph—a victory which might have been predicted, but the time for which did not seem yet to have come.

Every one is acquainted with the spectrum which we see when a ray of sunlight coming through a narrow opening passes through a prism. With the aid of suitable instruments there can also be seen in this spectrum a considerable number of dark cross-lines; and science has shown that these lines are caused by the presence of certain simple bodies or elements, including iron, hydrogen, sodium, etc. When we examine the light of the stars through those instruments, we shall perceive that in their spectrums too the dark lines denoting these elements are present. On this is founded the chemistry of the stars, for which we are wholly indebted to spectrum analysis. The situation of the dark lines in the spectrum is unchangeable, or else we would not be able to conclude from it respecting the elements represented there. The unchangeable character persists, however, only when the source of light is at rest as to the observer. If the shining body we are regarding is going away from us very rapidly, the dark lines incline to shift themselves slightly toward the red end of the spectrum; while, if it is approaching us with great rapidity, they slide over toward the violet. Without stopping to explain the causes of the shifting, we may remark that it is very small even with the greatest velocities. Former observers could hardly recognize it with certainty, because their instruments were not delicate enough to reveal such slight changes. Gradually makers have succeeded in constructing instruments that will show the changes with certainty. At the Greenwich Observatory, where observations of this kind have been carried on for several years, the motions in space of several stars have been ascertained with the help of the spectroscope. It has thus been found that the clear-shining Capella is receding from the earth at the rate of twenty-seven English miles a second, and that the brilliant star Vega in Lyra is approaching us at the rate of thirty-four miles a second. As such observations deal with infinitesimally small magnitudes, they are necessarily very difficult and precarious. It has been found, by investigations at the Astrophysical Observatory in Potsdam, that much more certain results are obtained if the spectrums of the stars are photographed and the measurements of the lines are made afterward on the pictures. These results have been confirmed by spectro-photographic researches at the Cambridge Observatory in North America; and thus the spectrographic method justifies the