ASTRONOMY. 369 



have probably been frequently observed by experienced spectroscopists 

 without any bright lines being detected in them, whilst a false appear- 

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

 circumstauces, it would appear hazardous to accept Mr. Sherman's re- 

 sult without further evidence." {Nature, December 17, 1885.) 



Periodic variations in the spectrum of ft I/yrw. — Herr von Gothard re- 

 ported last year {Bull. Astron., i, p. 211) the appearance of the brilliaut 

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

 ■opeiae and ft Lyrse. During the year he has been able to prove the 

 periodic variability of the line D3 in the spectrum of ft Lyrse. The period 

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

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

 y Cassiopeiae also ofiers some indications of analogous variations. 



PROPER MOTION OF STARS. 



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

 ti'Science, vi, 358), combined with its large proper motion (4"-]0), brings 

 lit into prominence as the third or fourth of the stars moviug rapidly 

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

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

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

 Jline 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 ju, 

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



«ight — 



vt = ali 



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 — 



t; = 2-93if 

 n 



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

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

 correspondingly uncertain. The authorities for the adopted values of it 

 are given in the column following them. In the case of 40 o^ Eridani, 

 ■\\e 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 two comparison stars. The latest values of Hall and Ball 

 for 01 Cygni are practically identical. The i)robable errors of all the 

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

 H. Mis. 15 24 



