ts 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.]

[Footnote 309: Cassegrain, a Frenchman, substituted in 1672 a _convex_
for a _concave_ secondary speculum. The tube was thereby enabled to be
shortened by twice the focal length of the mirror in question. The great
Melbourne reflector (four feet aperture, by Grubb) is constructed upon
this plan.]

[Footnote 310: _Phil. Trans._, vol. civ., p. 275, _note_.]

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