496 Scientific Proceedings, Royal Dublin Society. 



number to 40 per cent. Now, is there the smallest probability that 

 a similar per centage of the stars of any other magnitude will be 

 found to possess a sensible parallax ? I think not. When fainter 

 stars have been examined for parallax they have been almost 

 always selected for large proper motion, or some other supposed 

 indication of nearness, and were not therefore fair specimens of 

 their class. But what do we find even as regards these selected 

 stars ? Of four hundred and nine, examined by Dr. Ball at Dun- 

 sink, two only gave a sensible parallax. Granted that a parallax, 

 equal to that of Sirius or Wega, might have escaped observation 

 on the methods employed by Dr. Ball, still the fact that only -i 

 per cent, of the specially selected stars turned out to have a sensi- 

 ble parallax is, I think, rather a startling one. No doubt astrono- 

 mers were at first a little surprised at finding that several stars of 

 the sixth magnitude have a sensible parallax ; but, as the stars of 

 that magnitude are one hundred and sixty times as numerous as 

 those of the first magnitude, it would require one hundred and 

 sixty stars with sensible parallaxes to compensate for a Centauri 

 alone. There seems to be no chance of detecting one-fourth of 

 that number. The evidence afforded by researches on parallax 

 hitherto is therefore, I think, entirely in favour of the smaller 

 average distance of the brighter stars. Individual faint stars are 

 sometimes nearer than individual bright ones ; but this does not 

 affect the question when we are dealing with averages. 



From parallax I pass to proper motion. I think there is no 

 doubt that the great proper motion of some faint stars is mainly 

 due to the fact that the actual velocity of their motions is very 

 great. Grounds might, perhaps, be alleged for holding that the 

 average velocity of small stars is greater than that of large ones ; 

 but, at all events, there is no more reason to believe that all stars are 

 moving with the same velocity than that all stars are equally bright 

 at the unit of distance. The test of proper motion, however, has 

 a defect from which that of brightness is free. Besides the dis- 

 tance of the star and the velocity of its motion ; the direction of the 

 motion must also be taken into consideration, for, if the star is 

 moving towards us or away from us, the proper motion will appear 

 insignificant, although the distance may not be great, and the 

 actual velocity of motion may be considerable. However, the evi- 

 dence of proper motion is, I think, likewise favourable to the 



