J. C. KAPTEYN VAN MAANEN 



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must have to do with two star streams, parallel to the lines joining 

 our solar system and the two points mentioned. 



It is evident that such a discovery as that of the star streams 

 would revolutionize the ideas of the structure of the universe. But 

 at the same time it pointed out the necessity of collecting an increas- 

 ing amount of data, in order to secure more reliable measures and 

 especially data for the fainter stars. It was clear that the desire 

 for such data could be satisfied only by the thorough cooperation 

 of several institutions according to a well-organized plan. Kapteyn 

 certainly was the right man to start such an organization. Through 



Fio. 2. — Distribution of proper motion in different parts of the sky which led to Kapteyn's 

 discovery of the star streams 



his work he had come into contact with most of the leading astron- 

 omers all over the world. His visits to America in 1904 and South 

 Africa in 1905 gave splendid opportunity for discussing his plans 

 with a number of eminent astronomers. In order to enable those 

 who showed an interest in the matter to judge more thoroughly 

 of the details, Kapteyn worked out a provisional plan; the result 

 was a great deal of discussion and many useful suggestions. In 

 1906 Kapteyn published his famous Plan of Selected Areas. This 

 pamphlet gives briefly but clearly, as only Kapteyn could give 

 it, a program for the further attack on the structure of the uni- 

 verse. It includes not only the general plan but also in careful 



