556 ANNUAL EEPOBT SMITHSONIAN INSTITUTION, 1923 



parallaxes of only 34 stars were known, the best results being due to 

 heliometer observations, especially by Gill and Elkin at the Cape of 

 Good Hope. 



When Kapteyn came to Groningen his appointment was to the 

 professorship in astronomy, calculus of probabilities, and theoretical 

 mechanics, but he found no observatory at his disposal. Good mathe- 

 matician as Kapteyn was, his heart was drawn more toward the 

 practical side of his science, and during the first years in Groningen 

 he tried hard to secure funds for an observatory, with a 6-inch helio- 

 meter as its principal instrument. In the beginning his efforts seemed 

 to promise success, and ground for the observatory was bought a little 

 outside of the city, but funds for the erection of the buildings were 

 not forthcoming until many years later, by which time Kapteyn in his 

 unique astronomical laboratory had founded an establishment which 

 satisfied, better than an observatory could have done, the needs of 

 this wonderful combination of the practical and the theoretical 

 astronomer. 



Lack of an opportunity for observational work was, however, 

 keenly felt by Kapteyn during the early years of his professorship, 

 and he requested Prof. H. G. van de Sande Bakhuyzen to let him use 

 the meridian circle of the Leiden Observatory during his vacations. 

 The request was granted and Kapteyn planned a careful program for 

 the observation of stellar parallaxes; he introduced the differential 

 method of observations in right ascension, thus deriving parallaxes 

 for 15 stars, which in accuracy competed with those yielded by the 

 heliometer, while the observations required less time. His thorough 

 discussion of the method and of these observations in Volume VII of 

 the Annals of the Leiden Observatory is one of the many contributions 

 from his hand which will be recorded among the classics of astronomy. 



But it was clear that for a solution of his great problem parallaxes 

 must be determined more rapidly. In 1889, at the conference of 

 the Carte du Ciel, Kapteyn outlined an ingenious scheme for meas- 

 uring the parallaxes of a large number of stars by means of photog- 

 raphy. The plan is extremely simple in theory : On the same plate 

 three exposures are made at the epoch of maximum parallactic dis- 

 placement; half a year later, at the minimum, six other exposures 

 are made on the same plate, and three again at the following maxi- 

 mum; after development the plate shows 12 images of each star 

 which in practice are arranged as follows : 



Max. Min. Max. 



Fig. 1. — Arrangement of exposures for the determination of stellar parallaxes according 



to Kapteyn's plan 



