370 Proceedings of the Royal Society of Edinburgh. [Sess. 
been recognised that the Sun’s motion among the stars could be seen 
reflected in a systematic spreading or closing of the fields towards or away 
from which the Sun was moving. Kapteyn discovered that, apart from this, 
there was a polarity in the proper motion which might be understood as 
dividing the stars considered, into two great clusters, one moving across 
the other. This wholly unexpected revelation, which has since been 
amply confirmed, gave a new orientation to research, showing, as it did, 
the possibility of separating the stellar problem into manageable portions, 
and of thinking in terms of groups and clusters of stars, the common 
properties of which were discoverable. 
Kapteyn’s writings are often mathematical, but with mathematics 
severely subordinated to his actual problem, which is usually of a 
statistical or numerical character. Though not himself an observer, he 
kept touch with observation by annual visits, for many years, to the 
great observatory of the Carnegie Institution at Mount Wilson, California, 
of which he was a Research Fellow. His simple-hearted enthusiasm 
endeared him to many friends. He was well known to British astronomers, 
and was an Associate of the Royal Astronomical Society and a Foreign 
Member of the Royal Society of London, as well as of our own Society. 
He visited Edinburgh on the occasion of the meeting of the British 
Association in 1921, when the University conferred upon him the Honorary 
Degree of LL.D. He died on 18th June 1922. 
