PROBLEMS OF ASTROPHYSICS 455 



dents of the solar corona and of the gaseous nebulae are discussing 

 the properties of the hypothetical elements coronium and nebulium 

 almost as familiarly as if they had actually handled them. Out of 

 some 20,000 absorption lines mapped by Rowland, more than the 

 half are awaiting laboratory identification. 



In this connection, the mathematical relations existing between 

 the positions of lines in the spectra of many of the principal elements, 

 discovered by Balmer, 1 Kayser, 2 Runge, and Paschen, 3 have already 

 been of great utility; and they can scarcely fail to illuminate the 

 question of the construction of the atoms involved. 



A new era of physical science was inaugurated about eight years 

 ago by the discovery of argon on the one hand, and of the X-rays on 

 the other. The former was followed by the discovery, in quick suc- 

 cession, of several other constituents of the earth's atmosphere which 

 at present demand our attention as to their presence in chromospheric 

 and auroral phenomena. It would be most surprising if the many 

 forms of radiation, including those of the radioactive substances, dis- 

 covered in the train of the X-rays, should not throw strong light upon 

 the constitution of matter. And how shall we deal intelligently with 

 the forms of matter in other worlds before we understand the constitu- 

 tion of matter upon the earth? The modern theory of electrons, in 

 which material atoms play the subordinate part, and electric charges 

 the principal part, promises to have a wide application to celestial 

 phenomena. Further, the actual transport and interchange of matter 

 in the form of small particles, from one star to another, as urged with 

 great learning and skill by Arrhenius, 4 seems to be a plain and un- 

 avoidable consequence of recently established physical facts. Should 

 this theory stand the test of time, its far-reaching consequences 

 would accord it a position of the first rank. 



The photographic programme inaugurated with the Crossley Re- 

 flector by Keeler comprised 104 negatives of the regions containing 

 the principal nebula? and star-clusters. These photographs, covering 

 but one six-hundredth part of the entire sky, record 850 5 nebulae, of 

 which 746 are new. If this proportion should hold good over the 

 whole sphere, the number discoverable with this instrument, with 

 exposures of ordinary length, would be half a million. This estimate 

 would be too large in case the smaller nebulas have a tendency to 

 cluster around the prominent nebulae, which to some extent is prob- 

 ably true. The number of stars visible in our great telescopes is of 

 the order of one hundred millions. The dark or invisible bodies in- 



1 Annalen der Physik, xxv, 80. 



2 Berlin Abhandlungen, 1890. 



3 Annalen der Physik, 1897, LXI, 641. 



4 Proceedings, Royal Society, LXXIII, 496. 

 6 Lick Observatory Bulletin, no. 64. 



