Present State ofKncmledge of the Parallax of the Stars. 599 



Of these memoirs, it is sufficient to say that Mr. Henderson's 

 parallax of a Centauri is discussed at some length ; and the result 

 is, that there seems a strong probability of a sensible parallax in 

 this remarkable star, which is strongly recommended to the atten- 

 tion of southern astronomers. 



Lastly, in discussing Bessel's parallax of 61 Cygni, the author 

 proceeds as follows * : — 



That the indication of a parallax in the agreement between the 

 fourth and fifth columns of the preceding tables may be rendered 

 more evident, a graphical projection of them is added. The time 

 being set off by proportional spaces on the line of the abscissae, in- 

 clined lines are drawn through the origin (Jan. 1, 1838), whose 

 ordinates represent the effects of Bessel's correction of the proper 

 motion with contrary signs. From these lines the above differences 

 are set off in the direction of ordinates to the abscissa. The curve, 

 therefore, which passes through the extremities of these ordinates, 

 represents the periodical effect of parallax ; and accompanying 

 curves being given, exhibiting the true effect of Bessel's assumed 

 value of the constant, the agreement between the two is shown to 

 be most complete. In the case of measures of distance from star 

 (a), the maximum and minimum and vanishing ordinates of the 

 curve are shown with almost as much regularity by the observed 

 differences as by the assumed parallax ; and in the second case, 

 though the agreement is not so close, yet the general law of the 

 curve of sines is well preserved. 



In arguing on the evidence afforded by the foregoing tables and 

 graphical illustrations, the author concludes, not only that a real 

 parallax has been detected, but that its amount is very approxi- 

 mately given in Bessel's investigation, who is enabled, by repeating 

 the same process, to diminish at pleasure the residual errors of the 

 determination. This feature separates completely this from all 

 former attempts, in some few of which an amount, rather greater 

 than the limiting probable errors, would seem to announce a paral- 

 lax, of which the evidence is yet so slight as to leave the mind quite 

 unsatisfied of its existence ; while the uncertainty of its amount 

 (supposing its existence to be proved) prevents its application to 

 ulterior objects in sidereal astronomy. 



To the memoir are annexed two appendices, the first of which 

 contains the investigation of formulae for computing the coefficients 

 of the constant of parallax in the two cases ; and also for finding 

 the variation in the angle of position of two stars very near each 

 other, one of which is affected by parallax. 



The second contains a translation of the most important parts 

 of Bessel's description of his Heliometer, from the Astronomische 

 Nachrichtcn, vol. viii. No. 189. 



An extract was read from a letter from Professor Bessel to the 

 President, stating that the observation on 61 Cygni had been con- 



• Bessel's letter is contained in vol. iv. No. 17, of the Notices of the 

 Astronomical Society [and in Phil. Mag., Third Series, vol. xiv. p. 68.]. 



