ASTRONOMY. 



1C7 



with regard to a suitable eompaiison star, Mliicli for the teleseope and 

 luicroineter used should not bo more than 300" distant from the star 

 whose parallax is to be examined, and not fainter than the 10th 

 magnitude. 



By comparing the two observations referred to above it can be de- 

 termined whether the object under examination has a parallax of any 

 considerable amount or not. Of course it would have been preferable 

 to make the observations at times when the elfect of parallax on the 

 relative positions of the star under examination and the comparison 

 star would be greatest ; but the determination of these times would 

 have entailed some additional labor, and Professor Ball has been satis- 

 iied with the simpler process described. 



Whenever the observations have suggested the existence of a meas- 

 urable parallax, the objects have been observed again, and when neces- 

 sary a complete series of observations has been made. 



As might be expected, the results of the present work are mainly of 

 a negative character; still the observations will be useful to future par- 

 allax seekers, and are also valuable as a small instrument of that com- 

 plete survey of the heavens which is still a desideratum. The observa- 

 tions are, however, sufficient to enable us, in Professor Ball's opinion, to 

 assert that the parallax of these stars is certainly less than a second of 

 arc, and most probably less than half a second of arc. It is, of course, 

 possible that no star in the heavens has a parallax larger than a second. 

 Professor Ball somewhat naively remarks he often thinks that there 

 certainly is not. It must, however, be remembered that comparatively 

 very few stars have been examined with this object in view, and the 

 induction is founded on really a very small number of instances. 

 Professor Ball's parallax-hunting is therefore a class of work which 

 ought to be done, and it is most satisfactory to find that it is being done 

 so skillfully and well. 



In the present volume there are also included some positive fruits of 

 Professor Ball's work, viz, determinations of the parallax of 61 (B) 

 Cygui (from differences of declination), of Groombridge 1618, and of 6 

 (B) Cygni.* 



The following are the results : 



Star. 



Parallax. 



Majrni- 

 tudo of 

 compari- 

 son star. 



61(P.)Cv!mi 

 Groom. 1018 

 6 (U) Cygni. 



5.9 I +0". 4676 + 0". 0321 



+ 0".322 ±0".023 ' 8.8 



6.5 I +0".482 ±0".054 10.5 



These three stars have large proper motions. 



As these determinations have, we believe, been previously i)ublished 



*Flamsteed's G Cygui is B of tliat constellation ; the star Jiere referred to is Groom- 

 bridge 2789, and the appellation "0 Cygni" is taken from Bode. 



