THE POSSIBILITIES OF COOPERATION 399 



upon the volunteer efforts of cooperating astronomers in various 

 parts of the world. One of these astronomers assumes such 

 a task as the determination of the brightness of the stars, of 

 a certain range of magnitude, in the Selected Areas. Another 

 deals with their positions and motions, another with their 

 velocities measured with the spectroscope, etc. Each observer 

 is able to take a large number of Selected Areas, covering so 

 much of the sky that he may separately discuss the bearing of 

 his results on some important 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 

 advantage of observing stars for which many data, necessary 

 for his own purposes, have been obtained by other members 

 of the cooperating group. Outside the Selected Areas, such 

 data are usually lacking, because so small a proportion of 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 

 Astronomical Union. The spectrum of iron contains thou- 

 sands of lines, many of which are well adapted for use as 

 standards. The work of determining their positions was under- 

 taken by the members of an international committee, in accord- 

 ance with certain specifications formulated by_the Solar Union. 

 But those who took part in the investigation were not bound 



