1921-22.] 
Obituary Notices. 
369 
Jacobus Cornelius Kapteyn. By Professor R. A. Sampson, F.R.S. 
(Read January 8, 1923.) 
Jacobus Cornelius Kapteyn occupied a unique place in astronomy. 
Born on January 19, 1851, he was appointed in 1878 to the professorship 
of astronomy at the University of Groningen, a post which he held till 
his retirement in 1921, and which he made one of the most famous chairs 
in the world. There was no observatory. Under parallel circumstances, 
he might have occupied himself with mathematical developments, as many 
celebrated astronomers have done. In place of that he conceived and 
gradually perfected a new branch of discussion. The first phase of it 
was the measurement and reduction of the photographic survey of the 
southern heavens. The plates had been taken by Gill at the Cape 
Observatory, but they lay unusable because unreduced. The work of 
reduction was new, it required a man of first-rate ability, and was 
immensely laborious. Kapteyn volunteered to undertake it, and it occupied 
him for thirteen years. The Cape Photographic Durchmusterung, which 
resulted, contains measures of place and magnitude of 450,000 stars. 
Laborious as the task was, it gave Kapteyn a sureness of judgment in 
dealing with voluminous stellar material which could not perhaps have 
been otherwise acquired. It secured him permanently against unreality 
in discussing the documents of astronomy. He saw, perhaps the first, 
that the time was now ripe for a real attack upon the question. What is 
the Stellar Universe ? The Groningen publications are devoted to this 
question — or since, when Kapteyn began to write, we were very far from 
being in a position to give any answer, he set himself to collect such 
contributory particulars as were available, to criticise them, to point out 
what more was required, to devise means of evading some obstacle that 
could not be conquered, to select regions for intensive study that should 
give an intelligent statistical view of the whole, to define practicable 
problems, the solution of which could throw a light on dark places ; and 
so forth. His zeal and judgment made him eminently successful in this, 
and he gradually acquired an unquestioned and magisterial authority in 
the astronomical world. His contributions were not, however, confined 
to pointing out the path. In his searches through the material, he made 
many discoveries. The most famous of these, as it well deserves to be, 
is the discovery of the two Star Streams. From Herschel’s time it had 
VOL. XLII. 24 
