study the _idiosyncrasy_ of each one of the mechanical contrivances at his disposal, in order that its inevitable, but now certified deviations from ideal accuracy may be included amongst the numerous corrections by which the pure essence of even approximate truth is distilled from the rude impressions of sense. Nor is this enough; for the casual circumstances attending each observation have to be taken into account with no less care than the inherent or _constitutional_ peculiarities of the instrument with which it is made. There is no "once for all" in astronomy. Vigilance can never sleep; patience can never tire. Variable as well as constant sources of error must be anxiously heeded; one infinitesimal inaccuracy must be weighed against another; all the forces and vicissitudes of nature--frosts, dews, winds, the interchanges of heat, the disturbing effects of gravity, the shiverings of the air, the tremors of the earth, the weight and vital warmth of the observer's own body, nay, the rate at which his brain receives and transmits its impressions, must all enter into his calculations, and be sifted out from his results. It was in 1823 that Bessel drew attention to discrepancies in the times of transits given by different astronomers.[346] The quantities involved were far from insignificant. He was himself nearly a second in advance of all his contemporaries, Argelander lagging behind him as much as a second and a quarter. Each individual, in fact, was found to have a certain definite _rate of perception_, which, under the name of "personal equation," now forms so important an element in the correction of observations that a special instrument for accurately determining its amount in each case is in actual use at Greenwich. Such are the refinements upon which modern astronomy depends for its progress. It is a science of hairbreadths and fractions of a second. It exists only by the rigid enforcement of arduous accuracy and unwearying diligence. Whatever secrets the universe still has in store for man will only be communicated on these terms. They are, it must be acknowledged, difficult to comply with. They involve an unceasing struggle against the infirmities of his nature and the instabilities of his position. But the end is not unworthy the sacrifices demanded. One additional ray of light thrown on the marvels of creation--a single, minutest encroachment upon the strongholds of ignorance--is recompense enough for a lifetime of toil. Or rather, the toil is its own reward, if pursued in the lofty spirit which alone becomes it. For it leads through the abysses of space and the unending vistas of time to the very threshold of that infinity and eternity of which the disclosure is reserved for a life to come. FOOTNOTES: [Footnote 305: Grant, _Hist. Astr._, p. 527.] [Footnote 306: _Optica Promota_, p. 93.] [Footnote 307: _Phil. Trans._, vol. xxxii., p. 383.] [Footnote 308: _Ibid._, vol. xc., p. 65.]