AN ADDRESS ON ASTROPHYSICS. 3 11 



Harvard College Observatory has been remarkably successful in 

 discovering variable stars by means of peculiarities in their spectra, 

 as well as in classifying them, and in qualitative studies of many 

 spectral details, using objective-prism spectrographs; but it is hoped 

 that slit spectrographs, attached to powerful telescopes, may soon be 

 devoted systematically to this subject, as it constitutes one of the 

 richest fields now awaiting development. 



A century and a half of meridian-circle observations has given to 

 the world, as one of many priceless contributions, a knowledge of the 

 proper motions of several thousand stars. Some of the ablest astron- 

 omers have used these results as a basis for determining the most prob- 

 able elements of the sun's motion, and in studies upon the distribution 

 of the stars in space. Unfortunately, these investigations necessarily 

 involve assumptions as to the unknown distances of the stars. 



A few years following the application of the spectroscope to the 

 study of celestial objects, Huggins recognized that the Doppler-Fizeau 

 principle supplied, in theory at least, the long hoped-for method of 

 measuring the components of stellar motions in the line of sight — 

 their radial velocities; and that the application of this method would 

 enable us to determine both the direction and the speed of the solar 

 motion, entirely independently of the distances of the stars. Efforts 

 to apply this method met with signal failure for twenty years, and 

 doubts even as to ultimate success were quite generally felt and freely 

 expressed. The beginnings of success were made by Huggins and 

 Pickering, in showing that photography reveals, with great clearness, 

 the delicate spectral lines which the eye in purely visual observations 

 is unable to see at all. In 1888, Vogel applied this knowledge in the 

 first photographic attempt to measure radial velocities, and his work 

 inaugurated a new era. His observations, obtained with a small tele- 

 scope and imperfect spectrograph, were not sufficiently accurate to meet 

 the needs of the principal sidereal problems, but they led to several 

 brilliant discoveries at Potsdam, and were invaluable in marking out 

 the path of progress. It was not until 1896 that the use of a powerful 

 telescope, equipped with an efficient spectrograph, gave results accurate 

 enough to satisfy present requirements. In fact, the accuracy ob- 

 tained exceeded our most hopeful expectations. 



It is not surprising that thirty years were required to develop 

 successful methods. The work is so delicate that, unless suitable pre- 

 cautions are taken at every point in the process, the errors introduced 

 may readily be larger than the quantities sought for. With the Mills 

 spectrograph, for example, a speed of nine kilometers per second dis- 

 places the lines only 0.01 mm. The probable error of a velocity deter- 

 mination for the best stars, such as Polaris, is but one fourth of a 

 kilometer per second, corresponding to a linear displacement of 0.0003 



