Encyclopaedia Britannica, or, a Dictionary of arts, sciences, and miscellaneous literature : enlarged and improved. Illustrated with nearly six hundred engravings > Volume 1, A-AME

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berrr.tion. being able, by another inftrutnent, to con Si ra the truth
of the obfervatums hitherto made with that of Mr Mo-
lineux, was no fmall inducement to the undertaking ;
but the chief of all was, the opportunity he (hould
thereby have of trying in what manner other itars
Should be affefted by the fame caufe, whatever it. might
be. For Mr Molineux’s inftrument being originally
defigned for obferving y Draconis, to try whether it
had any fenfible parallax, it was fo contrived, as to be
capable of but little alteration in its dire&ion ; not a-
bove feven or eight minutes of a degree : and there be¬
ing but few ftars, within half that diftanee from the
zenith of Kew, bright enough to be well obferved, he
could not, with his inftrument, thoroughly examine
how this caufe affe&ed ftars that were differently Situ¬
ated, with refpeff to the equinoctial and folftitial points
of the ecliptic.
Thefe conftderations determined him •, and by the
contrivance and direction of the fame ingenious perfon,
Mr Graham, his inftrument was fixed up the 19th of
Auguft 1727. As he had no convenient place where
he could make ufe of fo long a telefcope as Mr Moli¬
neux’s, he contented himfelf with one of but little
more than half the length, namely of 12 feet and a half,
the other being 24 feet and a half long, judging from
the experience he had already had, that this radius
would be long enough to adjuft the inftrument to a
Sufficient degree of exaftnefs : and he had no reafon
afterwards to change his opinion ; for by all his trials
he was very well fatisfied, that when it was carefully
re&ified, its fituation might be fecurely depended on
to half a fecond. As the place where his inftrument
was hung, in fome meafure determined its radius; fo
did it alfo the length of the arc or limb, on which the
divifions were made, to adjuft it: for the arc could
not conveniently be extended farther, than to reach
to about 6-J- degrees on each fide of the zenith. This
however was fufficient, as it gave him an opportunity
of making choice of feveral ftars, very different both
in magnitude and fituation ; there being more than two
hundred, inferted in the Britiffi Catalogue, that might
be obferved with it. He needed not indeed to have
extended the limb fo far, but that he was willing to
take in Capelin, the only ftar of the firft magnitude
that came fo near his zenith.
His inftrument being fixed, he immediately began
to obferve fuch ftars as he judged moft proper to give
him any light into the caufe of the motion already
mentioned. There was a fufficient variety of fmall
ones, and not lefs than twelve that he could obferve
through all feafons of the year, as they were bright
enough to be feen in the day time, when neareft the
fun. He had not been long obferving, before he
perceived that the notion they had before entertained,
that the ftars were fartheft north and South when the
fun was near the equinoxes, was only true of thofe
ftars which are near the folftitial colure. And after
continuing his obfervations a few months, he difcover-
ed what he then apprehended to be a general law ob¬
ferved by all the ftars, namely, that each of them be¬
came ftationary, or was fartheft north or South, when
it paffed over his zenith at fix of the clock, either in
the evening or morning. He perceived alfo that what¬
ever fituation the ftars were in, with refpedl to the car¬
dinal points of the ecliptic, the apparent motion of
4
every one of them tended the fame way, when they Aberration.
pafled his inftrument about the fame hour of the cay
or night ; for they all moved Southward when they
pafied in the day, and northward when in the night j
So that each of them was fartheft north when it came
in the evening about fix of the clock, and fartheft fouth
when it came about fix in the morning.
Though he afterwards difeovered that the maxima,
in moft of thefe ftars, do not happen exaftly when they
pafs at thofe hours ; yet, not being able at that time
to prove the contrary, and fuppofing that they did, he
endeavoured to find out what proportion the greateft
alterations of declination, in different ftars, bore to
each other ; it being very evident that they did not all
change their inclination equally. It has been before
noticed, that it appeared from Mr Molineux’s obfer-
vatinns, that y Draconis changed its declination above
twice as much as the before-mentioned fmall ftar that
was nearly oppofite to it ; but examining the matter
more nicely, he found that the greateft change in the
declination of thefe ftars, was as the fine of the lati¬
tude of each ftar refpe&ively. This led him to fufpeft
that there might be the like proportion between the
maxima of other ftars ; but finding that the obferva¬
tions of fome of tfyern would not perfectly correfpond
with fuch an hypothefis, and not knowing whether the
fmall difference he met with might not be owing to
the uncertainty and error of the obfervations, he de¬
ferred the farther examination into the truth of this hy¬
pothefis, till he Should be furniftied with a feries of ob¬
fervations made in all parts of the year ; which would
enable him not only to determine what errors the ob¬
fervations might be liable to, or how far they might
fafely be depended on ; but alfo to judge, whether
there had been any fenfible change in the parts of the
inftrument itfelf.
When the year was completed, he began to exa¬
mine and compare his obfervations; and having fatif-
fied himfelf as to the general laws of the phenomena,
he then endeavoured to find out the caufe of them.
He was already convinced that the apparent motion
of the ftars was not owing to a nutation of the earth’s
axis. The next that occurred to him, was an altera¬
tion in the dire£lion of the plumb-line, by which the
inftrument was. conftantly adjufted; but this, upon
trial, proved infufficient. Then he confidered what
refraction might do ; but here alfo he met with no fa-
tisfaftion. At lart, through an amazing fagacity, he
conjeftured that all the phenomena hitherto mention¬
ed, proceeded from the progreftive motion of light,
and the earth’s annual motion in her orbit : for he
perceived, that if light were propagated in time, the
apparent place of a fixed objeCt would not be the
fame when the eye is at reft, as when it is moving in
any other direCHon but. that of the line paffing through
the objeCI and the .eye ; and that when the eye is mo¬
ving in different dire&ions, the apparent place of the
objeCt would be different. {Hutton's Math. DiEl.').
Aberration, in Optics, the deviation or difperflon
of the rays of light, when reflected by a fpeculum,
or refraCted by a lens, which prevents them from
meeting or uniting in the fame point, called the geo¬
metrical focus, but are fpread over a fmall fpace, and
produce a confufion of images. There are two fpecies
of aberration diftinguiffied by their different caufes;
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