Royal Astronomical Society. 563 



apprehension, and so leading him on in hopeless, endless, and ex- 

 hausting pursuit. 



The pursuit, however, though eager and laborious, has been far 

 from unproductive even in those stages where its immediate object 

 has been baffled. 



The fact of a periodical fluctuation of some kind in the apparent 

 places of the stars was recognized by Flamsteed, and erroneously 

 attributed to parallax. The nearer examination of this phenomenon 

 with far more delicate instruments, infinitely greater refinement of 

 method, and clearer views of the geometrical relations of the sub- 

 ject, rewarded Bradley with his grand discoveries of aberration and 

 nutation, and enabled him to restrict the amount of possible parallax 

 of the stars observed by him within extremely narrow limits. 



Bradley failed to detect any appreciable parallax, though he con- 

 sidered 1" as an amount which would not have escaped his notice. 

 And since his time this quantity has been assumed as a kind of 

 conventional limit, which it might be expected to attain but hardly 

 to surpass. But this was rather because, in the best observations 

 from Bradley's time forward, 1" has been a tolerated error; a quan- 

 tity for which observation and mechanism, joined to atmospheric 

 fluctuations and uncertainties of reduction, could not be held -rigidly 

 accountable even in mean results; than from any reason in the 

 nature of the case, or any distinct perception of its reality. If 

 parallax were to be detected at all by observations of the absolute 

 places of the stars, it could only emerge as a" residual pheno- 

 menon," after clearing away all the effects of the uranographical 

 corrections as well as of refraction, when it would remain mixed up 

 with whatever uncertainties might remain as to the coefficients of 

 the former, with the casual irregularities of the latter, and with all 

 the forms of instrumental and observational error. Now these 

 have hitherto proved sufficient, even in the observation of zenith 

 stars, quite to overlay and conceal that minute quantity of which 

 astronomers were in search. 



It is not my intention, gentlemen, to enter minutely into the 

 history of the attempts of various astronomers on this problem, 

 whether by the discussion of observations of one star, or by the 

 combination of those of pairs of stars opposite in right ascension ; 

 nor with the occasional gleams of apparent success which, however, 

 have always proved illusory, which have attended these attempts. 

 For such a history, and, indeed, for a complete and admirably 

 drawn up monograph of the whole subject, I must refer to a paper 

 lately read to this Society by Mr. Main, and which is now in pro- 

 cess of publication in the forthcoming volume of our Memoirs *. 

 In whatever reference I may have to make to the history of the 

 subject, I must take this opportunity to acknowledge my obligations 

 to the author of this paper, as well as for his exceedingly luminous 

 exposition of the results of those more successful attempts on the 



[* See Phil. Mag., Third Series, vol xviii. p. 597-— Ed.]. 



