required a transit instrument and mural circle with a more powerful
telescope. Airy combined the functions of both, and employed the same
constructors as before to make a _transit-circle_ with a telescope of
eleven and a-half feet focus and a circle of six-feet diameter, the
object-glass being eight inches in diameter.

Airy, like Bradley, was impressed with the advantage of employing
stars in the zenith for determining the fundamental constants of
astronomy. He devised a _reflex zenith tube_, in which the zenith
point was determined by reflection from a surface of mercury. The
design was so simple, and seemed so perfect, that great expectations
were entertained. But unaccountable variations comparable with those
of the transit circle appeared, and the instrument was put out of use
until 1903, when the present Astronomer Royal noticed that the
irregularities could be allowed for, being due to that remarkable
variation in the position of the earth's axis included in circles of
about six yards diameter at the north and south poles, discovered at
the end of the nineteenth century. The instrument is now being used
for investigating these variations; and in the year 1907 as many as
1,545 observations of stars were made with the reflex zenith tube.

In connection with zenith telescopes it must be stated that Respighi,
at the Capitol Observatory at Rome, made use of a deep well with a
level mercury surface at the bottom and a telescope at the top
pointing downwards, which the writer saw in 1871. The reflection of
the micrometer wires and of a star very near the zenith (but not quite
in the zenith) can be observed together. His mercury trough was a
circular plane surface with a shallow edge to retain the mercury. The
surface quickly came to rest after disturbance by street traffic.