24 G REPORTS OX THE STATE OF SCIENCE 



Stellar Distribution and Movements. By A. S. Eddington, 



ilf.il., M.Sc. 



[Ordered by the General Committee to be printed in extenso.] 



The last few years and perhaps especially the last twelve months have 

 brought to light many new facts bearing on the question of stellar dis- 

 tribution and movements. The advance must be attributed principally 

 to the greatly improved data that have recently become available for dis- 

 cussion. When the data of proper motions, of radial velocities, and 

 of parallaxes now at hand are compared with what existed ten years 

 ago, there is found an evident reason for what might otherwise appear 

 to be a sudden outbreak of activity in this branch of research. We are 

 now to take stock of the new facts and of the old, and try to see to what 

 conception of the structure of the universe they lead; whilst new 

 results are continually being obtained and theories are in a fluid state, 

 this is a task of some difficulty ; but it is at a time like the present 

 that a general discussion of the subject may be especially useful. 



Let me begin by presenting an outline of the universe as it is 

 revealed by modern researches; the details will follow later. First, 

 it is believed that the great mass of the stars, excluding the Milky Way, 

 are arranged in the form of a lens or bun-shaped system. Our sun 

 occupies a nearly central position, or at least a position midway between 

 the two flattened surfaces. The thickness of this system, though 

 enormous when compared with ordinary units, is not so great but that 

 our telescopes easily detect the absence of stars beyond. We cannot 

 specify the thickness definitely, because there is no definite boundary, 

 but only a gradual thinning out in the number of stars. The plane of 

 the lens-shaped system is the same as the plane of the Milky Way, so 

 that when we look towards the galactic poles we are looking towards 

 the parts where the boundary is nearest to us ; looking along the galactic 

 plane, we are looking towards the perimeter of the lens, where the 

 boundary (or thinning out of the stars) is most remote, though prob- 

 ably not beyond the penetrating power of our telescopes. 



Near the sun the stars seem to be distributed in a fairly uniform 

 manner, or rather, there are irregularities, but they are on a small 

 scale; but in the remoter part of the lens, or perhaps right beyond it, we 

 come across the great cluster of series of star-clouds which make up the 

 Milky Way itself. I think that is a right distinction. There are two 

 quite separate phenomena associated with the galactic plane which have 

 sometimes been confused. Firstly it is the plane in which the Milky 

 Way star-clouds are coiled, and secondly it is the median plane of the 

 lens-like arrangement of the nearer stars. 



