442 SCIENCE PROGRESS 



as though a fall of snow had collected into huge irregular drifts 

 and then a shower of rain fell : the number of raindrops falling 

 on a given area would have nothing to do with the quantity 

 of snow upon it already, and we should only make confusion by 

 expressing the rain as a percentage of the snow on the area. 

 Similarly we must give up the idea of finding so many sensible 

 movements per cent, of the stars on a given area and think of 

 the number per unit area, irrespective of the population by other 

 stars. The moving stars are in fact distinct from the others 

 in some way, and it is pretty clear that the distinction consists 

 in their being considerably nearer to us. If this be the correct 

 interpretation, we infer that the stars near us are scattered more 

 or less uniformly and do not show the structure which is so 

 striking a feature of those more distant. This is a fact of funda- 

 mental importance and suggests from a new point of view the 

 idea of a solar cluster — a group of stars of which our sun is one 

 standing somewhat apart from its distant surroundings — which 

 was suggested by the counts of stars of different magnitudes as 

 described in the last article. 



And its importance is enhanced in consequence of a great 

 discovery made within the last few years, that there are two 

 great streams of stars meeting one another. It w^as assumed 

 until about 1904 that the movements of the stars were at 

 random in all directions ; in that year Prof. Kapteyn of 

 Groningen showed that this could not be the case,i but that 

 there must be at least two main streams of stars passing one 

 through the other. This suggestion has been confirmed by 

 elaborate investigations of Eddington, Dyson and others : and 

 in particular Prof. Dyson (now Astronomer Royal) showed 

 that this bifurcation was specially characteristic of the largest 

 proper motions : that is to say, the stars nearest to our sun 

 are moving in this way in any case, whatever may be the real 

 facts about the more distant stars of which our knowledge 

 is still uncertain and incomplete. Is there any reason for 

 thinking that the bifurcation characterises only the nearest 

 stars and ceases beyond ? One such reason has already been 

 indicated : the nearest stars are apparently distinct from those 

 more remote which cluster towards the Milky Way. Hence 



* A few months later Mr. H. C. Plummer independently pointed out the same 

 fact. {Mo?L Not. R.A.S. vol. Ixv. p. 568.) 



