October 1, 1915] 



SCIENCE 



439 



ing, brighter than the fifth magnitude, 

 Fainter stars are now being observed. At 

 the Mount "Wilson Observatory, Professor 

 Adams has determined the velocities of 

 stars of known parallaxes, as there are great 

 advantages in obtaining complete data for 

 stars where possible. Extension of line-of- 

 sight determinations to fainter stars is sure 

 to bring a harvest of useful results, and a 

 number of great telescopes are engaged, and 

 others will shortly join in this important 

 work. 



PROPER MOTIONS 



As proper motions are determined by the 

 comparison of the positions of stars at two 

 different epochs, they get to be known with 

 constantly increasing accuracy as the time 

 interval increases. The stars visible to the 

 naked eye in the northern hemisphere were 

 accurately observed by Bradley in 1755. 

 Many thousands of observations of faint 

 stars down to about 9.0 m. were made in 

 the first half of the nineteenth century. An 

 extensive scheme of reobservation was car- 

 ried out about 1875 under the auspices of 

 the Astronomische Gesellschaft. A great 

 deal of reobservation of stars brighter than 

 the ninth magnitude has been made this 

 century in connection with the photographic 

 survey of the heavens. For the bright stars 

 all available material has been utilized and 

 their proper motions have been well deter- 

 mined, and for the fainter stars this is being 

 gradually accomplished. 



Proper motions differ widely and irreg- 

 ularly in amount and direction. Herschel 

 observed a tendency of a few stars to move 

 towards one point of the sky, and attributed 

 this sign of regularity to a movement of the 

 solar system in the opposite direction. But 

 puzzling differences given by different 

 methods remained unexplained until the 

 difficulty was resolved by Professor 

 Kapteyn in a paper read before this section 

 of the British Association at its meeting in 



South Africa ten years ago. He showed 

 that the proper motions had a general tend- 

 ency towards two different points of the sky 

 and not towards one only, as would be ex- 

 pected if the motions of the stars themselves 

 were haphazard, but viewed from a point 

 in rapid motion. He concluded from this 

 that there was a general tendency of the 

 stars to stream in two opposite directions. 

 It is interesting to notice that this great 

 discovery was made by a simple graphical 

 examination of the proper motions of stars 

 in different regions of the sky, after the 

 author had spent much time in examining 

 and criticizing the different methods which 

 had been adopted for the determination of 

 the direction of the solar motion. The sub- 

 ject was brought into a clearer and more 

 exact shape by the analytical formulation 

 given to it by Professor Eddington, and. 

 after him by Professor Schwarzschild. 



This star-streaming is corroborated by 

 observations of velocities in the line of 

 sight. It applies — with the exception of 

 the helium stars — to all stars which are 

 near enough for their proper motions to be 

 determinable. We may say with certainty 

 that it extends to stars at distances of two 

 or three hundred parsecs; it may extend 

 much further, but I do not think we have at 

 present much evidence of this. Professor 

 Turner pointed out that the convergence of 

 proper motions did not necessarily imply 

 movements in parallel directions, and sug- 

 gested that the star-streams were movements 

 of stars to and from a center. The agreement 

 of the radial velocities with the proper mo- 

 tions seems to me to be opposed to this sugges- 

 tion, and to show that star-streaming indi- 

 catesapproximate parallelism in two opposite 

 directions in the motions of the stars exam- 

 ined. As the great majority of these stars 

 are comparatively near to us, it is possible 

 that this parallelism is mainly confined to 

 them, and indicates the general directions 



