The Spedvum of ol Aquilce. 



283 



imitated by holding a rather faint drawing, of the solar spectrum at 

 such a distance that the individual lines are no longer visible. 



From a consideration of the photographs taken at Harvard College 

 Observatory, Professor Pickering suggested in 1891 that the diffuse- 

 ness of the lines in the spectrum of a Aquilse and certain other 

 stars was perhaps due to a rapid rotation of the star.* That rotation 

 might be capalile of producing such effects had already been suggested 

 by Abney in 1877.t 



Photographs taken at Kensington with large dispersion, during 1892, 

 led to the adoption of Professor Pickering's view, and the spectrum of 

 a Aquila3 was classed with that of p Arietis, in which we apparently 

 got the same lines quite sharp. J 



In 1895 Dr. Scheiner again referred to this spectrum,§ and sug- 

 gested that it represents a transition stage from the first to the second 

 type ; as an alternative explanation, he mentions the view that the 

 spectrum may be a composite one, in which a spectrum of the first 

 type is superposed upon one of the second. 



In a recent paper|| Dr. Yogel has discussed the spectrum of a Aquilse 

 chiefly with reference to its motion in the line of sight, but he also 

 considers the question of the haziness of the lines. He refers to some 

 experimental photographs which depict the solar spectrum with its 

 lines broadened by a cylindrical lens, or by a photograph taken out of 

 focus, and states that spectra of this kind have been obtained in which 

 the close lines run together so as to produce a spectrum resembling 

 that of OL Aquilse. He adds that the exact comparison of the two 

 spectra shows that the agreement is not perfect, in particular that the 

 G group is hardly indicated in the spectrum of a Aquilae, while it 

 comes out strongly in the solar spectrum when thrown out of focus. 

 He accordingly places the spectrum of a Aquilse in his Class la 3, of 

 which a Cygni, /3 Cassiopeise, and Procyon are members, H and further 

 concludes that the lines are broadened in consequence of rapid rotation, 

 without, however, referring to previous suggestions to the same effect. 



The general result is that while Vogel classes oc Aquilse with a Cygni, 

 which, on the meteoritic hypothesis, is a star of increasing tempera- 

 ture, the work at Kensington indicates that it should be classed with 

 stars like Arietis, which there is every reason to believe to be cool- 

 ing. This difterence as to facts is so important that the whole question 

 has been re-investigated. 



* 'Annals Hai-v. Coll. Obs.,' vol. 26 (1891), Pt. I, p. 21. 



t 'Monthly Notices R.A.S.,' vol. 37, p. 278. 



I 'Phil. Trans.,' A, vol. 184 (1893), p. 697. 



§ ' Pub. Ast. Obs. zu Potsdam,' vol. 7, Part II, p. 232. 



!! 'Sitzber. Akad. Berlin,' Nov. 1898; translated in ' Astropbys. Journ.,' Jan. 

 1899. 



IF 'Astrophys. Journ.,' vol. 2 (1895), p. 346. 



