ASTRONOMY. o69 



bare probably been frequently observed by experienced spectroscopists 

 witlioiit n7iy bright lines being detected in tbem, whilst a false appear- 

 ance of bright lines is readily produced in stellar spectra under certain 

 circumstances, it would ai>pear hazardous to accept Mr. Sbernian's re- 

 sult without further evidence." (Nafure, December 17, 1885.) 



Periodic I'ariations in the spectrum of /:/ Lyrcc. — Herr von Gotbard re- 

 porte<l last year [Bull. Astron,, i, p. 211) the appearance of the brilliant 

 lines, which had not been seen for some time, in the spectra of ^ Cassi- 

 opeiaj and fJ Lyr.ie. During the year he has been able to prove the 

 periodic variability of the line D3 in the spectrum of /i Lyroe. The period 

 seems to be very short, and is probablj^ about 7 days. The hydrogen 

 lines vary also, but their variation is less pronounced. The spectrum of 

 y Cassiopeiiii also offers some indications of analogous variations. 



PROPER MOTION OF STARS. 



Stars in rapid motion. — The small value of the parallax of 40 o^ Eridani 

 [Science, vi, 358), combined with its large })roper motion (4"-l()), brings 

 it into prominence as the third or fourth of the stars moving rapidly 

 across our line of sight. Since a Tist of these stars seldom appears in 

 works on popular astronomy, we give below the proper motions //, the 

 parallaxes tt, and the resulting velocities v, in miles per second across our 

 line of sight, of the eight stars which head the list in the order of veloci- 

 ties. The method of deriving the velocities is of course very simple. 

 If a star's annual proper motion equals its parallax, it moves across our 

 line of sight each year a distance equal to the semi-major axis of the 

 earth's orbit. (How much it moves to or from us can only be told by 

 the spectroscope.) Therefore, since this motion increases directly as //, 

 and inversely as ;r, we have for the annual motion across the line of 

 sight — 



vt = at!: 

 n 



or, calling a 92.5 million miles, and t the number of seconds in a year, 

 we have for the volocity in miles per second — 



^ = 2-93^^ 



71 



Of course, the proper motions below are much more accurately known 

 than the parallaxes, and where the latter are small the values of t are 

 correspondingly^ uncertain. The authorities for the adopted values of n 

 are given iu the column following them. In the case of 40 «- Eridani, 

 we have weighted Gill and Hall 2 and 1 respectively, as the former de- 

 termination was made under much the more favorable conditions, and 

 rests upon tw^o comparison stars. The latest values of Hall ami r>all 

 for 01 (Jygni are practically identical. The probable errors of all the 

 values of n are generally less than 0"'02. 

 H. Mis. 15 24 



