236 Sir Xormaii Lockyer and Mr. A. Fowler. 



with certainty. The place occupied by the manganese quartet 

 4030-88 — 4035*88) is covered in the spectrum of the star by what 

 appears to be a broad hazy line, which is probably composed of the 

 individual components of the quartet merged together. The arc line 

 of calcium at X 4226-90 is one of the most prominent lines in the 

 spectrum. 



The classification of the spectrum of a Aquilae may therefore be 

 considered as settled ; it does not sufficiently resemble a Cygni to 

 justify Yogel's view that it should be classed with that type of star, 

 while, on the other hand, apart from the haziness of the lines, it does 

 bear a very strong resemblance to /S Arietis and other Sirian stars, 

 and should therefore be classed with them. 



[Note, February 8. — In a later publication"^ Yogel places a Cygni in 

 his Class la 2, with Sirius, j3 Arietis, &c., but this does not materially 

 modify the conclusions arrived at.] 



There are other points on which this demonstration of the similarity 

 of cc Aquil^ and /5 Arietis may be brought to bear, among them being 

 the determination of the lines most suitable for the measurement of 

 the velocity of the star in the line of sight, and the approximate deter- 

 mination of the velocity of rotation necessary to produce the observed 

 haziness of the lines. 



Lines suitable for the Determination of the Velocity of the Star in the Line 



of Sight. 



For the measurement of the velocity of ol Aquilse, Deslandres has 

 employed comparison spectra of hydrogen, iron, and calcium, t Vogel, 

 however, questions the advantage of using the spectra of iron and 

 calcium as comparisons for this purpose, on the ground that " the lines 

 in the spectrum of a Aquilae are so diffuse .... that between Hjs 

 and Hs no lines except those of hydrogen and the magnesium line at 

 X 4481 can be identified with known lines." He himself has used the 

 Hy line alone as a term of comparison, and concludes that there are no 

 indications of a periodic change in the velocity of the star in the line 

 of sight, such as was supposed by Deslandres. 



We now know with certainty the origins of a considerable number 

 of the lines in a Aquilfe, so that measurements of the velocity of the 

 star are placed on a surer basis. Since the spectrum of oc Aquilae is 

 simpler than that of the sun, some of the broad lines do not represent 

 confused groups of lines, but are broadened individual lines. The latter 

 class of lines, when of known origin, seems to be well adapted for the 



* ' Pub. Ast. Obs. Potsdam,' 1890, vol. 12, Part I, p. 49. 

 t ' Comptes Eeiidus,' vol. 121 (189,")), p. 629. 



