May 26, iSSi] 



NATURE 



91 



a similar discontinuous spectram by mixing together any or all of 

 the bodies which alone cou'd survive the aforesaid chemical 

 treatment ; — when all these facts are taken into consideration, 

 and when due weight is given to the very characteristic spectrum 

 reaction, I cannot help concluding that the most probable 

 explanation is that these anomalies are caused by the presence of 

 an unknown body whose chemical reactions are not sufficiently 

 marked to have enabled chemists to differentiate it from associated 

 elements. 



THE DISTANCES OF THE STARS ^ 

 T7 VERY one who is acquainted with the rudiments of astronomy 

 knows that the sun with its attendant planets is merely an 

 island group in the va^t realms of space. 



An island the size of this room in the middle of the Atlantic 

 would hardly be more remotely apart from the surrounding 

 shores than is our solar system from the bodies which surround 

 it in space. To determine the distance from this solar system 

 to the stars which surround it is the problem for our consideration 

 to-night. 



Recent Researches on 61 Cygni. — It is now almost exactly forty 

 years (February 12, 1841) since the gold medal of the Royal 



Astronomical Society was awarded to Bessel for his discovery of 

 the annual parallax of 61 Cygni. On that occasion Sir John 

 Herschel delivered an address, in which he glanced at the labours 

 of Stru\e and of Henderson as well as those of Vessel. The 

 discovery of the distances of the stars was alluded to as "the 

 gi-eatest and most glorious triumph which practical astronomy 

 has ever witnessed." From this date the history of our accurate 

 know ledge of the subject may be said to commence. Each suc- 

 ceeding race of astronomers takes occasion to investigate the 

 parallax of 61 Cygni anew, with the view of confirming or of 

 correcting the results arrived at by Bessel. 



[The parallactic ellipse vvliich the stars appear to describe, 

 having been briefly explained, the method of deducing the 

 distances of the stars was pointed out.] 



The attention of Bessel was directed to 61 Cygni by its proper 

 motion of five seconds per annum. When Bessel undertook his 

 labours in 1S3S the pair of stars forming the double were in the 

 position indicated on Fig. I. When O. Struve attacked the 

 problem in 1S53 the pair of stars forming 61 Cy^ni had moved 

 considerably. Finally, when the star was observed at Dunsink 

 in 187S, it had made another advance in the same direction as 

 before. In forty years this object had moved over an arc of the 

 heavens upwards of three minutes in length. 



^ BESSEL 



.,70 



BESSEL 



I878; 



-Gi Cygni and parallax 



The diagram contains four other stars besides the three posi- 

 tions of 61 Cygni. These are but small telescopic objects, they 

 do not participate in the large proper motion of 61 Cygni, and 

 they are undoubtedly much more remote from us. Bessel 

 chose as the comparison stars the two objects marked with his 

 name. He measured the distance from the central point of 

 61 Cygni to each of the two comparison stars. From a series 

 of such measures he discovered the parallactic eUipse of 61 Cygni. 

 He was led to the same ellipse by each of the two comparison 

 stars. 



Fifteen years later (1853) Struve undertook a new detennina- 

 tion. He chose a comparison star different from either of those 

 Bessel had used. Stnive's method of observing was also quite 

 different from Bessel's. Struve made a series of measures of 

 the distance and position of the comparison star from 61 (B) 

 Cygni. Struve also succeeded in measuring the parallactic 

 ellipse. 



There was, however, an important difference between their 



' Lecture at the Royal Institution of Great Britain, on Friday February 

 II, by Prof. Robert S. Ball, LL.D,, F.R.S., Astronomer Royal of Ireland 



results. The distance, according to Bessel, was half as much 

 again as Struve found. Bessel Slid the distance was sixty 

 billions of miles ; Struve said it could not be more than forty 

 billions. 



The di-crepancy may be due to the comparison stars. If 

 Bessel's compari-on st.ars were only about three times as far as 

 61 Cyjni, while Stnive's star was about eight or ten times as 

 far, the difference between Struve's results and Bessel's would 

 be accounted for. 



To settle the question, observations were subsequently made 

 by Auwers and others ; the latest of these investigations is one 

 which has recently been completed at Dunsink Observatory. 



Dr. Briinnow proposed and indeed commenced a series of 

 meaures of the difference in declination between 61 Cygni and 

 a fourth comparison star. These observations were made with 

 the south equatorial at Dunsink. The carrying out of this work 

 devolved on the lecturer, as Dr. Bninnow's successor. Two 

 series of observations have been made, one with each of the 

 components of 6l Cygni. The results agree very nearly with 

 those of Struve. 



