AN ADDRESS ON ASTROPHYSICS. 3°5 



the auroras, — in fact of every phenomenon of nature involving minute 

 particles. And what celestial object does not involve them? 



On the other hand, the student of the stars has pointed the way 

 for the laboratory investigator, in many instances. The ultra-violet 

 hydrogen series was photographed by Huggins, in the spectrum of 

 Vega, before it was found in the laboratory; and Pickering has dis- 

 covered another hydrogen series, in Zeta Puppis, which still awaits 

 terrestrial duplication. The hypothetical element, helium, in the 

 sun, waited a quarter-century for Ramsay's discovery, and the labo- 

 ratory investigation of its more complete spectrum which followed. 

 Students 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 Eowland, 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, Kayser, Runge and Paschen, 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 radio-active 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 phe- 

 nomena. 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, seems to be a plain and unavoid- 

 able 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 program inaugurated with the Crossley Reflector 

 by Keeler comprised 104 negatives of the regions containing the prin- 

 cipal nebulas and star clusters. These photographs, covering but one 

 six-hundredth part of the entire sky, record 850 nebulas, of which 746 

 are new. If this proportion should hold good over the whole sphere, the 



