362 TRANSACTIONS OP SECTION A. 



was modified by Prof. Schwarzschild, who considered the stars, not as two 

 separate streams, but as exhibiting a polarity in their proper motions. It is 

 difficult to say which of the two harmonises better with the observations — 

 they agree in the most essential fact, that the stars have a very decided prefer- 

 ence for motion towards a point in the Milky Way situated in the constellation 

 of Ophiuchus and the opposite point in the constellation of Orion. 



This star-streaming is corroborated by ob.servations 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. Prof. Turner pointed out that the convergence 

 of proper motions did not necessarily imply movements in parallel directions, 

 and suggested that the star-streams were movements of stars to and from a 

 centre. The agreement of the radial velocities with the proper motions seems 

 to me to be opposed to this suggestion, and to show that star-streaming 

 indicates approximate parallelism in two opposite directions in the motions of 

 the stars examined. 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 of the orbital motions of stars in the neighbour- 

 hood. An attempted explanation on these lines, as on Prof. Turner's, implies 

 that the Sun is some distance from the centre of the stellar system. 



A discovery of an entirely different character was made by Prof. Boss in 

 1908. He spent many years in constructing a great catalogue giving the most 

 accurate positions and motions of 6,200 stars obtainable from all existing 

 observations. This catalogue, which was published by the Carnegie Institute, 

 was intended as a preliminary to a still larger one which would give the 

 accurate positions and motions of all the stars down to the seventh magnitude. 

 In the introduction to the catalogue. Boss remarks that this collation of the 

 results of meridian observations in a large and comprehensive way is only the 

 second attempt which has yet been made by astronomers. The first, it is 

 interesting to notice, was a general catalogue of 8,377 stars compiled by 

 Francis Baily and published by the British Association in 1845. At that time 

 the proper motions could only be given for a very limited number of stars, but 

 in Boss's catalogue proper motions are given for all the stars, and their pro- 

 bable errors are not more than 0"'5 per century. In the course of this work 

 Professor Boss found that forty or fifty stars scattered over a considerable 

 region of the sky near the constellation Taurus were all moving towards the 

 same point in the sky and with nearly the same angular velocity. He inferred 

 that these stars were all moving in parallel directions with an equal linear 

 velocity, and the supposition was verified, in the case of several of them, by 

 the determination of their radial velocities. From these data he was able to 

 derive the distance of each star and thus its position in space. The existence 

 of a large group of stars, separated from one another by great distances, and 

 all having the same motion in space, is a very remarkable phenomenon. It shows, 

 as was pointed out by Prof. Eddington, how small is the gravitational action 

 of one star on another, and that the movement of each star is determined by 

 the total attraction of the whole mass of the stars. Several other interesting 

 moving clusters have been found since. For all the stars belonging to these 

 clusters, the distances have been found, and from them luminosities and 

 velocities of individual stars, particulars which are generally only obtainable 

 for stars much nearer to us. 



Proper motions are the main sour-ce of our knowledge of the distances of 

 stars which are beyond the reach of determination by annual parallax. If a 

 star were known to be at rest its distance could be calculated from the 

 shift of its apparent position caused by the translation of the Solar motion. 

 As the Solar system moves 410 times the distance of the Earth from the Sun 

 in a century, this gives a displacement of 1" for a star at the distance of 

 500 parsecs. This method has been applied by Kapteyn to determine the 

 distances of the helium stars, as their velocities are sufficiently small to be 

 neglected in comparison with that of the Solar system. But generally it is 

 only possible to find the mean distances of groups of stars of such size that it 



