the auroræ,—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 discovered 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 laboratory investigation of its more complete spectrum which followed. Students of the solar corona and of the gaseous nebulæ 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, 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 succession, 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, discovered 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 constitution 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, seems to be a plain and unavoidable 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 principal nebulæ and star clusters. These photographs, covering but one six-hundredth part of the entire sky, record 850 nebulæ, of which 746 are new. If this proportion should hold good over the whole sphere, the
