Ball — On Stars ivith a Large Annual Parallax. 215 



XXXVI. — Obseevations in Seaech of Stars with a Laege Anntjai, 

 Paeallax. By Kobeet S. Ball, LL.D., F.R.S., Royal Astrono- 

 mer of Irelancl. 



[Read December 9, 1878.] 



In continuing the researches made by my predecessor Dr. Briinnow, 

 on the Parallax of Stars, I have adopted two different classes of 

 observation. The first of these is the ordinary continuous series of 

 observations of two or three specially chosen objects extending over 

 an entire year. This is, no doubt, the only method by which a parallax 

 amounting to a small portion of a second can be detected, much less 

 accurately measured. It is, however, to be observed, that as a set of 

 measures takes at least an hour to complete, it is almost impossible 

 for the most assiduous observer to have more than three stars in hand 

 at the same time. I have, therefore, adopted the course of having 

 two stars in regular observation at the same time, and of devoting 

 whatever other opportunities I may have to the system of observa- 

 tions which will be described in the present Paper. The full details 

 of the observations here referred to will shortly appear in Part III. of 

 the Dunsink Observations. 



It is, of course, well known that up to the present no parallax has 

 been detected which exceeds a single second of arc. In the great 

 majority of cases the parallax is very much less, even if it be appreci- 

 able. But when we reflect that not one star out of every ten 

 thousand has yet been regularly examined for parallax, it is obvious 

 that it would be in the highest degree rash to conclude that there are 

 no stars nearer to us than any of those of which we already know the 

 distance. 



In selecting objects appropriate for investigation of annual parallax, 

 astronomers have generally chosen those stars which are exceptional, 

 either on account of their brilliancy, or the largeness of their proper 

 motions. The presence of these exceptional features in a star is 

 doubtless a prima facie presumption that the star is comparatively 

 near us. On the other hand, it cannot but be observed that the 

 brightest star (i.e. Sirius) appears to have a parallax of only 0"'232,^ 

 while for the star Groombridge, 1830, which has the enormous proper 

 motion of 7"-05 annually, Briinnow has found a parallax no greater 

 than 0"-090. The presumptions of nearness founded on great bril- 

 liancy or great proper motion can hardly be said to be justified by the 

 results of observation. 



1 From Houreau's invaluable Repertoire des Constantes de PAstronomie I 

 extract the following :— " Jacq. Cassini, by the method of absolute altitudes in 1717, 

 found a paraUax of 6" for Sirius. Piarri, by the same method, in 1805 reduced 

 this to 4". Henderson, in 1840, from the meridian altitudes at the Cape, found the 

 value 0"-23. Gylden, in 1864, from the altitudes found by Maclear, at the Cape, 

 in 1836-7, deduced a parallax of 0"-193. Abbe, in 1868, from the altitudes at the 

 Cape, in 1856-1863, deduced the parallax 0"-273. A mean of the three last 

 determinations is the value given above." 



