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SCIENCE 



[N. S. Vol. LI. No. 1311 



omer, resolved many years ago to make a 

 serious effort to deal with the question. In 

 order to do so, as he had no telescope or other 

 observational means of his own, he enlisted 

 the cooperation of astronomers scattered over 

 the whole world. 



In organizing his attack, he recognized that 

 the inclusion of only the brighter stars, or 

 even of all those contained in the Interna- 

 tional Chart of the Heavens, would not nearly 

 suffice for his purpose. He must penetrate as 

 far as possible into the depths of space, and 

 therefore hundreds of millions of stars are of 

 direct importance in his studies. Moreover, 

 it is evident that if he were to confine his 

 attention to some limited region of the sky, 

 he could form no conclusions regarding the 

 distribution of stars in other directions in 

 space or such common motions as might be 

 shown, for example, by immense streams of 

 stars circling about the center of the visible 

 universe. 



As the measurement of the positions, the 

 motions, the brightness, and the distance of 

 all the stars within the reach of the most 

 powerful telescopes would be a truly Utopian 

 task. Professor Kapteyn wisely limited his 

 efforts, and at the same time provided a 

 means of obtaining the uniformly distributed 

 observations essential to the discussion of his 

 great problem. His simple plan was to divide 

 the entire sky into a series of 206 selected 

 areas, thus providing sample regions, uni- 

 formly spaced and regularly distributed over 

 the entire sphere. Conclusions based upon 

 the observation of stars in these areas are 

 almost as reliable, so far as large general 

 questions of structure and motion are con- 

 cerned, as though data were available for 

 all the stars of the visible sidereal universe. 



As already remarked. Professor Kapteyn de- 

 pends entirely upon the volunteer eilorts of 

 cooperating astronomers in various parts of 

 the world. One of these astronomers assumes 

 such a task as the determination of the bright- 

 ness of the stars, of a certain range of magni- 

 tude, in the selected areas. Another deals 

 with their positions and motions, another with 

 their velocities measured with the spectro- 



scope, etc. Each observer is able to take a 

 large number of selected areas, covering so 

 much of the sky that he may separately dis- 

 cuss the bearing of his results on some im- 

 portant problem, such as the distribution of 

 the stars of each magnitude with reference to 

 the plane of the Galaxy, the motions in space 

 of stars of different spectral types, the velocity 

 and direction of the sun's motion in space, the 

 dependence of a star's velocity upon its mass. 

 Moreover, each observer is free to use his 

 utmost ingenuity in devising and applying 

 new methods and instruments, in increasing 

 the accuracy of his measures, and in adopting 

 improved means of reducing and discussing 

 his observations. He also enjoys the advan- 

 tage of observing stars for which many data, 

 necessary for his own purposes, have been ob- 

 tained by other members of the cooperating 

 group. Outside the selected areas, such data 

 are usually lacking, because so small a pro- 

 portion (;f the total number of stars has been 

 accurately observed. 



In physics, as well as in astronomy, there 

 are innumerable opportunities for cooperative 

 research. A good illustration is afforded by 

 the determination of the exact wave-lengths 

 of lines in the spectra of various elements, 

 for use as standards in measuring the relative 

 positions of lines in the spectra of celestial 

 and terrestrial light-sources. This work was 

 initiated in 1904 by the International Union 

 for Cooperation in Solar Research, and is now 

 being continued by the International Astron- 

 omical Union. The spectrum of iron con- 

 tains thousands of lines, many of which are 

 well adapted for use as standards. The work 

 of determining their positions was undertaken 

 by the members of an international committee, 

 in accordance with certain specifications 

 formulated by the Solar Union. But those 

 who took part in the investigation were not 

 bound by any rigid rule. On the contrary, 

 they were encouraged to make every possible 

 innovation in the manner of attack, in order 

 that obscure sources of error might be dis- 

 covered and the highest possible accuracy in 

 the final results attained. The outcome dem- 

 onstrates most conclusively that organized 



