PRESIDENTIAL ADDRESS. 361 



can be no question of the substantial accuracy of these results, as they are 

 closely confirmed by discussions of proper motions. They are, however, very 

 difficult to understand. On the face of it, there does not seem any reason why 

 stars of a high temperature should have specially high velocities. A suggestion 

 has been thrown out by Dr. Halm that as the helium stars have greater masses, 

 these results are in accordance with an equi-partition of Energy. But the 

 distance of stars apart is so great that it seems impossible that this could be 

 brought about by their interaction. Prof. Eddington suggests that the 

 velocities may be an indication of the part of space at which the stars were 

 formed {e.g., stars of small mass in outlying portions), and represents the 

 kinetic energy they have acquired in arriving at their present positions. 



The stars whose radial velocities have been detei'niined are, generally speak-, 

 ing, brighter than the fifth magnitude. Fainter stars are now being observed. 

 At the Mount Wilson Observatory, Prof. Adams has determined the velocities 

 of stars of known parallaxes, as there are great advantages in obtaining com- 

 plete 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. 



Pro'peT 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 Qm-O were made 

 in the first half of the nineteenth century. An extensive scheme of re- 

 observation was carried 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 

 utilised, and their proper motions have been well determined, and for the 

 fainter stars this is being gradually accomplished. 



Proper motions differ widely and irregularly 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. As the amount of material increased, the question 

 was examined in different ways by Bessel, Argelander, and Airy. Bessel's 

 method did not indicate the Solar motion, while Airy's showed it plainly. 

 The cause of this discrepancy was not explained for more than half a century. 

 The publication by Auwers of very accurate proper motions of the stars 

 observed by Bradley, consisting roughly of 3,200 stars visible to the naked eye 

 in these latitudes, caused a number of astronomers to make fresh determina- 

 tions of the direction of the Solar motion. But the puzzling differences given 

 by different methods remained unexplained till the difficulty was resolved by 

 Prof. 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 tendency towards two different points of the sky and 

 not towards one only, as would be expected if the motions of the stars them- 

 selves were haphazard, but viewed from a point in rapid motion. He con- 

 cluded 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 criticising the different methods which had been adopted for 

 the determination of the direction of the Solar motion. 



The subject was brought into a clearer and more exact shape by the 

 analytical formulation given to it by Prof. Eddington. He emplo.yed the 

 proper motions of some 4,000 stars determined from the comparison of Groom- 

 bridge's observations in 1810 with modern observations at Greenwich. These 

 stars are moie suitable than Bradley's for analytical treatment, as there are 

 a larger number of them per unit area of the sky. This analytical treatment 



