124 



SCIENCE. 



[N. S. Vol. XVII. No. 421. 



universe. The steps to a conclusion on the 

 subject are briefly these: First we have 

 a general conclusion, the basis of which I 

 have already set forth, that, to use a loose 

 expression, there are likenesses throughout 

 the whole diameter of the universe. There 

 is, therefore, no reason to suppose that the 

 region in which our system is situated dif- 

 fers in any essential degree from any other 

 region near the central portion. Again, 

 spectroscopic examinations seem to show 

 that all the stars are in motion, and that we 

 cannot say that those in one part of the 

 universe move more rapidly than those in 

 another. This result is of the greatest 

 value for our purposes, because, when we 

 consider only the apparent motions, as 

 ordinarily observed, these are necessarily 

 dependent upon the distance of the star. 

 "We cannot, therefore, infer the actual 

 speed of a star from ordinary observations 

 until we know its distance. But the re- 

 sults of spectroscopic measurements of 

 radial velocity are independent of the dis- 

 tance of the star. 



But let us not claim too much. We can 

 not yet say with certainty that the stars 

 which form the agglomerations of the 

 Milky Way have, beyond doubt, the same 

 average motion as the stars in other regions 

 of the universe. The difficulty is that these 

 stars appear to us so faint individually, 

 that the investigation of their spectra is 

 still beyond the powers of our instruments. 

 But the extraordinary feat performed at 

 the Lick Observatory of measuring' the 

 radial motion of 1830 Groombridge, a star 

 quite invisible to the naked eye, may lead 

 us to hope for a speedy solution of this 

 question. But we need not await this result 

 in order to reach very probable conclusions. 

 The general outcome of researches on 

 proper motions tends to strengthen the con- 

 elusions that the Keplerian sphere, if I may 

 use this expression, has no very well 

 marked existence. The laws of stellar 



velocity and the statistics of proper mo- 

 tions, while giving some color to the view 

 that the space in which we are situated is 

 thinner of stars than elsewhere, yet show 

 that, as a general rule, there are no great 

 agglomerations of stars elsewhere than in 

 the region of the Milky Way. 



With unity there is always diversity; in 

 fact the unity of the universe on which I 

 have been insisting consists in part of 

 diversity. It is very curious that, among 

 the many thousands of stars which have 

 been spectroscopically examined, no two are 

 known to have absolutely the same physical 

 constitution. It is true that there are a 

 great many resemblances. a Centauri, 

 our nearest neighbor, if we can use such a 

 word as 'near' in speaking of its distance, 

 has a spectrum very like that of our sun, 

 and so has Capella. But even in these 

 cases careful examination shows differ- 

 ences. These differences arise from variety 

 in the combinations and temperature of 

 the substances of which the star is made 

 up. Quite likely also, elements not known 

 on the earth may exist in the stars, but 

 this is a point on which we cannot yet 

 speak with certainty. 



Perhaps the attribute in which the stars 

 show the greatest variety is that of absolute 

 luminosity. One hundred years ago it was 

 naturally supposed that the brighter stars 

 were the nearest to us, and this is doubtless 

 true when we take the general average. 

 But it was soon found that we cannot con- 

 clude that because a star is bright, there- 

 fore it is near. The most striking example 

 of this has been brought out by the re- 

 searches of Gill on the parallax of Rigel, 

 the brightest star in Orion, and of Canopus, 

 which is, next to Sirius, the brightest star 

 in the heavens. In both these cases the 

 parallax from a long series of measure- 

 ments, extending through several years, 

 came out just zero. These stars, then, 

 though of the first magnitude, are immeas- 



