1907-8.] The Systematic Motions of the Stars. 
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XIII. — The Systematic Motions of the Stars. By Professor Dyson. 
(MS. received February 3, 1908. Read February 17, 1908.) 
Many attempts have been made to interpret the systematic proper motions 
of the stars as the result of a movement of the solar system in space. In 
recent years several elaborate and extensive investigations have been made, 
but the increase of the available material in amount and accuracy has 
not led to a corresponding increase in the accuracy of the so-called “ Solar 
Apex.” Instead, determinations from different groups of stars have shown 
larger differences than can be attributed to accidental error, and in par- 
ticular a determination by Kobold, using Bessel’s method, has given results 
at variance with those obtained by Airy’s method. 
Professor Kapteyn was led to an examination of the fundamental 
hypothesis underlying all these investigations — viz., the solar motion apart, 
the proper motions of the stars show no preference for any special directions. 
Kapteyn found that the apparent proper motions of the stars (i.e. as seen by 
us, and therefore relative to the Sun) show drifts in two directions, and not 
in one only, as would be the case were the motion of the solar system the 
cause. Kapteyn’s results, derived from an examination of 2500 stars 
observed by Bradley about 1755, and repeatedly re-observed in modern 
times, are given in a short paper in the British Association Report for 1905 
(pp. 257-265). These results were confirmed by Mr Eddington ( Monthly 
Notices of the Royal Astronomical Society, vol. Ixvii. pp. 34-63) fro,m the 
examination of the proper motions of over 4000 stars within 52° of the 
North Pole, which were observed by Groombridge about 1810, and re- 
observed at Greenwich about 1890. 
The hypothesis that the stars are moving in two streams is of a revolu- 
tionary character, and calls for further investigation. In the following 
paper I have examined stars with large proper motions, the limits chosen 
being from 20" to 80" a century. The reason for choosing such stars is, that 
if the stars are moving in two streams, these streams would be indicated 
more strongly by excluding the stars of small proper motion. Again, limit- 
ing the stars in this manner, we eliminate the effect of errors of observation 
to a great extent, and may use stars whose proper motions are less 
accurately known than those of Bradley and Groombridge — for an error of 
2" or 3" a century, though important in the case of small proper motion, 
is of little consequence when the proper motion exceeds 20". 
