catalogued; and he
counted 108 such objects clustering round the star 31 Comae
Berenices,[1573] and so closely that all might be occulted together by
the moon. The general photographic Catalogue of Nebulae which Dr. Wolf
has begun to prepare[1574] will thus be a most voluminous work.

The history of celestial photography at the Cape of Good Hope began with
the appearance of the great comet of 1882. No special apparatus was at
hand; so Sir David Gill called in the services of a local artist, Mr.
Allis of Mowbray, with whose camera, strapped to the Observatory
equatoreal, pictures of conspicuous merit were obtained. But their
particular distinction lay in the multitude of stars begemming the
background. (See Plate III.) The sight of them at once opened to the
Royal Astronomer a new prospect. He had already formed the project of
extending Argelander's "Durchmusterung" from the point where it was left
by Schoenfeld to the southern pole; and his ideas regarding the means of
carrying it into execution crystallised at the needle touch of the
cometary experiments. He resolved to employ photography for the purpose.
The exposure of plates was accordingly begun, under the care of Mr. Ray
Woods, in 1885; and in less than six years, the sky, from 19 deg. of south
latitude to the pole, had been covered in duplicate. Their measurement,
and the preparation of a catalogue of the stars imprinted upon them,
were generously undertaken by Professor Kapteyn, and his laborious task
has at length been successfully completed. The publication, in 1900, of
the third and concluding volume of the "Cape Photographic
Durchmusterung"[1575] placed at the disposal of astronomers a
photographic census of the heavens fuller and surer than the
corresponding visual enumeration executed at Bonn. It includes 454,875
stars, nearly to the tenth magnitude, and their positions are reliable
to about one second of arc.

The production of this important work was thus a result of the Cape
comet-pictures; yet not the most momentous one. They turned the scale in
favour of recourse to the camera when the MM. Henry encountered, in
their continuation of Chacornac's half-finished enterprise of ecliptical
charting, sections of the Milky Way defying the enumerating efforts of
eye and hand. The perfect success of some preliminary experiments made
with an instrument constructed by them expressly for the purpose was
announced to the Academy of Sciences at Paris, May 2, 1885. B