J. C. KAPTEYN VAN MAANEN 557 



The distances a and b are then measured and reduced by a simple 

 process ; and yield, with respect to the mean parallax of all the stars 

 measured, the parallax of every star visible on the plates. The 

 method was put into practice by Kapteyn and Donner and the first 

 results were published in 1900 as No. 1 of the Publications of the 

 Astronomical Laboratory at Groningen, a remarkable series of pub- 

 lications which has contributed much to the development of astron- 

 omy in the last 20 years. 



While similar results for different fields, mostly in collaboration 

 with De Sitter, appear in the Groningen Publications, we soon per- 

 ceive a change in his policy of attack on the general problem. The 

 change from parallaxes to proper motion, however, is more apparent 

 than real, and is founded on the practical fact that by using the 

 proper motions we can base the parallaxes on the ever-increasing 

 base line of the sun's motion through space, and on the theoretical 

 fact that for the structure of the universe it is not at all necessary 

 to know the distances of individual stars, but the mean distances of 

 groups of stars for different magnitude, spectral type, and galactic 

 latitude. The problem has two requirements: An accurate determi- 

 nation of the sun's motion through space and a knowledge of the dis- 

 tribution of proper motion for an increasingly great number of 

 fainter and fainter stars. Along both lines the Groningen Publica- 

 tions reveal how much Kapteyn advanced our knowledge. And it 

 is in just such work as this that Kapteyn's double aptitude for recog- 

 nizing great problems and at the same time perceiving the practical 

 difficulties was of the greatest usefulness. Kapteyn would work out 

 a new method which, with the proper material, would give the de- 

 sired results; but he would at the same time also apply his method 

 to the material available, even when it was scanty and likely to 

 yield only defective results. This, however, had the advantage of 

 showing at once where the method itself could be improved and what 

 data would be most needed. We see him follow this means of attack 

 in all his problems, by successive steps coming ever closer to the 

 laws governing the structure of the universe. 



Incidentally, the investigations on proper motion led Kapteyn 

 to his discovery of the two star streams, which, rightly, was recently 

 selected by Eddington as one of the five greatest astronomical events 

 of the last hundred years, a discovery which has revolutionized 

 our ideas of the structure of the universe. In deriving the solar 

 motion Kapteyn was struck by the divergency of the results of 

 former investigators. In these researches it was usually assumed 

 that the motus peculiaris of the stars was at random, a natural 

 hypothesis, since with the enormous distances of the stars from one 

 another it was difficult, to see why there should be any relation be- 

 tween the individual motions of different stars. Yet stars moving 



