Sir W. M. H. Christie, Eighth Astronomer Royal, took up his duties in
that capacity in 1881. Besides a larger altazimuth that he erected in
1898, he has widened the field of operations at Greenwich by the
extensive use of photography and the establishment of large
equatoreals. From the point of view of instruments of precision, one
of the most important new features is the astrographic equatoreal, set
up in 1892 and used for the Greenwich section of the great
astrographic chart just completed. Photography has come to be of use,
not only for depicting the sun and moon, comets and nebulae, but also
to obtain accurate relative positions of neighbouring stars; to pick
up objects that are invisible in any telescope; and, most of all
perhaps, in fixing the positions of faint satellites. Thus Saturn's
distant satellite, Phoebe, and the sixth and seventh satellites of
Jupiter, have been followed regularly in their courses at Greenwich
ever since their discovery with the thirty-inch reflector (erected in
1897); and while doing so Mr. Melotte made, in 1908, the splendid
discovery on some of the photographic plates of an eighth satellite of
Jupiter, at an enormous distance from the planet. From observations in
the early part of 1908, over a limited arc of its orbit, before
Jupiter approached the sun, Mr. Cowell computed a retrograde orbit and
calculated the future positions of this satellite, which enabled
Mr. Melotte to find it again in the autumn--a great triumph both of
calculation and of photographic observation. This satellite has never
been seen, and has been photographed only at Greenwich, Heidelberg,
and the Lick Observatory.

Greenwich Observatory has been here selected for tracing the progress
of accurate measurement. But there is one instrument of great value,
the heliometer, which is not used at Greenwich. This serves the
purpose of a double image micrometer, and is made by dividing the