PROBLEMS OF ASTROPHYSICS 461 



For more than half a century a great many astronomers have de- 

 voted themselves assiduously to making photometric observations of 

 variable stars. There are a dozen observatories, both large and small, 

 which are systematically devoting some of their resources to this 

 work. By common consent of the profession, or by appointment 

 from learned societies, there have for some fifty years been individual 

 astronomers, or committees of astronomers, who systematize results, 

 call attention to the need for observations of certain neglected ob- 

 jects, and in many other ways encourage the photometric study of 

 variable stars. Photometers are inexpensive, the methods are simple, 

 and results have rapidly accumulated. 



Observations of variable stars with slit-spectrographs, on the 

 contrary, are surprisingly meagre and fragmentary. Not a single 

 institution, not a single telescope, not a single observer, is working 

 continuously or even extensively on the subject. Yet the method is 

 a very powerful one: the few isolated studies made on variable stars 

 have led to results of remarkable richness. The subject is one of great 

 difficulty. Photographic spectra require much time for accurate 

 measurement and reduction. And, finally, powerful and expensive 

 instruments are demanded. 



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 astro- 

 nomers have used these results as a basis for determining the most 

 probable elements of the sun's motion, 1 and in studies upon the dis- 

 tribution 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 

 1 Clerke's System of the Stars, chapter 23. 



