there is no trouble with refraction in the zenith. For these reasons
Bradley had set up this instrument at Kew, to attempt the proof of the
earth's motion by observing the annual parallax of stars. He certainly
found an annual variation of zenith distance, but not at the times of
year required by the parallax. This led him to the discovery of the
"aberration" of light and of nutation. Bradley has been described as
the founder of the modern system of accurate observation. He died in
1762, leaving behind him thirteen folio volumes of valuable but
unreduced observations. Those relating to the stars were reduced by
Bessel and published in 1818, at Konigsberg, in his well-known
standard work, _Fundamenta Astronomiae_. In it are results showing the
laws of refraction, with tables of its amount, the maximum value of
aberration, and other constants.

Bradley was succeeded by Bliss, and he by Maskelyne (1765), who
carried on excellent work, and laid the foundations of the Nautical
Almanac (1767). Just before his death he induced the Government to
replace Bird's quadrant by a fine new mural _circle_, six feet in
diameter, by Troughton, the divisions being read off by microscopes
fixed on piers opposite to the divided circle. In this instrument the
micrometer screw, with a divided circle for turning it, was applied
for bringing the micrometer wire actually in line with a division on
the circle--a plan which is still always adopted.

Pond succeeded Maskelyne in 1811, and was the first to use this
instrument. From now onwards the places of stars were referred to the
pole, not to the zenith; the zero being obtained from measures on
circumpolar stars. Standard stars were used for giving the clock
error. In 1816 a new transit instrument, by Troughton, was added, and
from this date the Greenwich star places have maintained the very