THE STRUCTURE OF THE UNIVERSE 393 



present day. It is evident that the data which we have to go 

 on is as yet insufficient. In a recent volume, 1 and in a 

 later monograph, Simon Newcomb, himself the foremost living 

 student of this especial subject, sums up the slender conclusions 

 which seem warranted thus far. He believes that the collec- 

 tion of the stars which we call the universe is limited in extent. 

 The smallest stars that we see with the most powerful tele- 

 scopes are not necessarily more distant than those of a grade 

 brighter, but are rather smaller or less luminous stars. This 

 does not preclude the possibility that far outside of our universe 

 there may be other collections of stars of which we know 

 nothing. 



The boundary of this especial universe is apparently some- 

 what indefinite and irregular. As we go outwards towards the 

 boundaries, the stars may thin out gradually. It does not seem 

 possible to decide whether the agglomerations of the Milky 

 Way lie on this^boundary or not. 



Seeliger of Munich would go a little further. This astronomer 

 made an exhaustive study of the distribution of the great mass 

 of stars relative to the so-called Galactic Plane. But the inquiry 

 extended only to stars of the ninth magnitude that is to say, 

 to a few hundred thousand out of, possibly, hundreds of millions. 

 Within these limits there does seem an unmistakable increase 

 of star density in the region of the Milky Way, and a progressive 

 decrease in either direction perpendicular to this plane. In 

 other words, these hundreds of thousands of brighter suns 

 seem collected into some such a mass as that imagined by 

 Herschel and Struve. In Seeliger's view, " the Milky Way is 

 no merely local phenomenon (local to us), but is closely con- 

 nected with the entire constitution of our stellar system." 



This idea is somewhat strengthened by the researches of 

 Celoria, the successor of Schiaparelli, at Milan. Celoria utilised 

 a very much larger number of stars ; his results were much 

 the same. But one circumstance is to be noted. From the 

 star gauges made by the Herschels, father and son, the crowd- 

 ing of the stars in the Galactic Plane appeared to be perhaps 

 twenty times more intense than towards the Galactic Poles 

 that is to say, in the regions at right angles to this plane. 



From the later studies of Celoria and others, it does not 

 appear that this Galactic star density is more than two or three 



1 Newcomb, The Stars, 1902. 



