5 o2 THE POPULAR SCIENCE MONTHLY. 



mon 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, so- 

 dium, 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 spec- 

 trum 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 them- 

 selves 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 in- 

 struments 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 cer- 

 tain results are obtained if the spectrums of the stars are photo- 

 graphed 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 great- 



