518 Dr. Bohert S. Ball [Feb. 11, 



as to whether all our stars form one system. If an isolated body in our 

 system is to remain there for ever, the theory of gravitation imposes 

 the imperative condition, that the velocity of the body must not exceed 

 a certain amount. Assuming that the stars are 100,000,000 in number, 

 and that each star is five times as large as the sun, assuming also that 

 they are spread out in a thin layer of such dimensions that a ray of 

 light takes 30,000 years to pass it, Newcomb shows that the critical 

 velocity is 25 miles per second. 



As this is only the eighth-part of the velocity of Groombridge 

 1830, we are thus led to the dilemma that either the masses of the 

 bodies in our system must be much greater than we have supposed, or 

 Groombridge 1830 is a runaway star, which can never be controlled 

 and brought back. 



Search for Stars with Parallax. 



The lecturer has been engaged for some years at Dunsink Obser- 

 vatory in a systematic search for stars which have an appreciable 

 parallax. Up to the present about three hundred stars have been 

 examined. In the majority of cases each of these stars has been 

 observed only twice. The dates of the observations have been chosen 

 so as to render the effects of parallax as manifest as possible. It is 

 not of course expected that a small parallax of a few tenths of a second 

 could be detected by this means. 



The errors of the observations would mask any parallax of this 

 kind. It seems, however, certain that no parallax could have escaped 

 detection if it at all approached to that of a Centauri. 



The stars examined have been chosen on various grounds. It had 

 been supposed that some of the red stars were possibly among the 

 sun's neighbours, and consequently many of the principal red stars 

 were included in our list. No appreciable parallax has, however, 

 been detected in any of the red stars up to the present. Many of the 

 principal double stars are also included in the list. Other stars have 

 been added on very various grounds, among them may be mentioned 

 the Nova, which some time ago burst out in the constellation Cygnus, 

 and dwindled down again to a minute point. The earth's orbit, how- 

 ever, does not appear any larger when seen from Nova Cygni than 

 from any of the other stars on our list. 



Groomhridge 1618. 



We have, however, found one star which seems to have some claim 

 to attention as one of the sun's neighbours. The star in question is 

 Groombridge 1618. It lies in the constellation Leo, and is 6*5 mag- 

 nitude. Groombridge 1618 has a proper motion of 1"*4 annually. 

 From a series of measurements of its distance made on fifty-five 

 nights from a suitable comparison star, the parallax of Groombridge 

 1618 appeared to be about one-third of a second. As this seemed to 

 be a result of considerable interest, measures were renewed for a 

 second series of forty nights. The result of the second series con- 



