ed, the Astrographic Chart--a great
scheme of international co-operation by which it is hoped to leave as a
legacy for future centuries a record of the state of the sky in our age.
Such a record cannot be complete; for however faint the stars included, we
know that there are fainter stars which might have been included had we
given longer exposures to the plates. Nor can it be in other ways final or
perfect; however large the scale, for instance, on which the map is made,
we can imagine the scale doubled or increased many-fold. But the map will
be a great advance on anything that has hitherto been made, and some
account of it will therefore no doubt be of interest.

[Sidenote: Origin of the chart.]

We may perhaps begin with a brief historical account of the enterprise.
Photographs of the stars were taken many years ago, but only by a few
enthusiasts, and with no serious hope of competing with eye observations
of the sky. The old wet-plate photography was, in fact, somewhat unsuited
to astronomical purposes; to photograph faint objects a long exposure is
necessary, and the wet plate may dry up before the exposure is
concluded--nay, even before it is commenced, if the observer has to wait
for passing clouds--and therefore it may be said that the successful
application of photography to astronomy dates from the time when the dry
plate was invented; when it became possible to expose a plate in the
telescope for hours, or by accumulation even for days. The dry plate
remains sensitive for a long period, and if it is desired to extend an
exposure beyond the limits of one night, it is quite easy to close up
the telescope and return to the operations again on the next fine night;
and so on, if not perhaps indefinitely, at any rate so long as to
transcend the limits of human patience up to the present.

[Illustration:

VII.--GREAT COMET OF NOV. 7TH, 1882
(_From a photograph taken at the Royal Observatory, Cape of Good Hope._)]

[Sidenote: Comet of 1882.]

[Sidenote: Stars shown on the pictures.]

But to consider our particular project. We may assign, perhaps, the date
1882 as that in which it first began to take shape. In that year there was
a magnificent bright comet, the last really large comet which we, in the
Northern Hemisphere, have had the good fortune to see. Some of us, of
course, were not born at that time, and perhaps others who were alive may
nevertheless not have seen that comet; for it kept somewhat unco