The Spectrum of « Aquilce. 



237 



measurement of the velocity of the star, for though they may be still 

 somewhat wide, they are much less so than the lines of hydrogen. 

 Among these broadened individual lines are : — 



Iron (enhanced lines).— 4178-95, 4233'25, 4385-55, 4549-64. 



„ (arc lines).— 4045-90. 

 Titanium (enhanced lines).— 4417-98, 4443-98. 

 Strontium. — 4215*66. 



The enhanced line of magnesium at 4481-3 is usually sharply defined 

 in stellar spectra, but the fact that it is generally fluffy in the compari- 

 son spark disqualifies it for accurate measurements. 



The Velocity of Botation. 



Assuming that /S Arietis represents the spectrum of a Aquilas as it 

 would appear if the axis were directed towards the earth, we can get 

 a general idea of the velocity of rotation necessary to produce the 

 observed broadening of the lines. For this purpose lines which occur 

 in groups are obviously unsuitable, but we can utilise the lines to which 

 attention has just been drawn. Taking the enhanced line of iron at 

 A 4178*95, we find thafits thickness is increased from about two to 

 four tenth-metres, and this corresponds to a surface velocity of the star 

 at the equator of about 45 miles per second, supposing that the axis is 

 perpendicular to the line of sight. Similar measurements of the broad- 

 ening of the magnesium line 4481-3 yield a velocity of about 40 miles 

 per second. Since only a small portion of the surface of the star 

 could exhibit the effects of the maximum velocity, it is probable that 

 these values are too low, really representing the equatorial velocity of 

 rotation resolved along the line of sight with reference to a point some- 

 where between the limb and centre of the star. 



Dr. Vogel gives reasons in his paper for supposing the velocity of 

 rotation to be possibly 27 kilometres (16*8 miles) per second, but this 

 determination does not depend upon measurements of individual 

 lines. 



Geneml Conclusions. 



The investigation of the Kensington photographs of the spectrum of 

 a Aquilse has thus led to the following conclusions : — 



(1) Apart from the general haziness of the lines, the spectrum pre- 

 sents no unfamiliar features. 



(2) The spectrum is of the Sirian type, showing enhanced lines of 

 various metals, and a smaller number of arc lines. 



(3) A rapid rotation of the star, as first suggested by Pickering, 

 appears to be a simple and sufficient explanation of the peculiarities of 

 the spectrum. 



