Royal Astronomical Society. 567 



nature and construction of this instrument, especially when driven 

 by clock-work, almost every conceivable error which can affect a 

 micrometrical measure is destroyed, when properly used ; and the 

 precautions taken by M. Bessel in its use have been such as might 

 be expected from his consummate skill. The only possible appa- 

 rent opening for an annually fluctuating error seems to be in the 

 correction for temperature of its scale. But this correction has been 

 ascertained by M. Bessel by direct observation, in hot and cold 

 seasons, and applied. Nor could this cause destroy the evidence 

 arising from the simultaneous observation of the two companion 

 stars, since a wrong correction for temperature would affect both 

 their distances proportionally, leaving the apparent parallactic 

 movement still unaccounted for. 



The resulting parallax is an extremely minute quantity, only 

 thirty-one hundredths of a second ; which would place the star in 

 question at a distance from us of nearly 670,000 times that of the 

 sun ! * Such is the universe in which we exist, and, which we have 

 at length found the means to subject to measurement, at least in 

 one of its members, probably nearer to us than the rest. 



It becomes necessary for me now to refer to two series of re- 

 searches on this important subject, which have been held by your 

 Council to merit very high and honourable mention ; though neither 

 of them, separately, for reasons which I shall state, would have been 

 considered as carrying that weight of probability in favour of its con- 

 clusions, which would justify any immediate decision of the nature 

 which they have come to in the case of M. Bessel's. I allude to M. 

 Struve's inquiries, by the method of micrometric measures, into the 

 parallax of a Lyrse ; and to Mr. Henderson's, by that of meridian 

 observations, on the parallax of a Centauri. 



a Lyrse is accompanied by a very minute star, at the distance of 

 about 43". That this star is unconnected with a by any physical 

 relation, is clear from the fact ascertained by Sir James South and 

 myself, that it does not participate in the proper motion of the large 

 star. The mutual angular distance of these stars has been made 

 by M. Struve the subject of a very extensive series of micrometric 

 measures with the celebrated Dorpat achromatic, bearing this ob- 

 ject steadily in view, and working it out to a conclusion of the very 

 same kind, and, though materially inferior in the degree and na- 

 ture of its evidence to that of Bessel, yet certainly entitled to high 

 consideration. M. Struve's observations on this star, and for this 

 purpose, extend from Nov. 1835 to Aug. 1838, and are distributed 

 over sixty nights, averaging twenty per annum ; and from their 

 combination, according to the principle of probabilities, he con- 

 cludes a parallax of 0"*261. Mr. Main has subjected these obser- 

 vations to an analysis and graphical projection, precisely similar in 

 principle to those I have explained in the case of 61 Cygni. The 

 curves so projected have been subjected to your inspection, and 



* The orbit described by the two stars of 61 Cygni about each other 

 •will, therefore, be about 50 times the diameter of the earth's about the sun, 

 or 2§ times that of Uranus. 



