y its means
stars estimated as of the sixteenth magnitude clearly recorded their
presence and their places; and the enormous increase of knowledge
involved may be judged of from the fact that, in a space of the Milky
Way in Cygnus 2 deg. 15' by 3 deg., where 170 stars had been mapped by
the old laborious method, about five thousand stamped their images on a
single Henry plate.

These results suggested the grand undertaking of a general photographic
survey of the heavens, and Gill's proposal, June 4, 1886, of an
International Congress for the purpose of setting it on foot was
received with acclamation, and promptly acted upon. Fifty-six delegates
of seventeen different nationalities met in Paris, April 16, 1887, under
the presidentship of Admiral Mouchez, to discuss measures and organise
action. They resolved upon the construction of a Photographic Chart of
the whole heavens, comprising stars of a fourteenth magnitude, to the
surmised number of twenty millions; to be supplemented by a Catalogue,
framed from plates of comparatively short exposure, giving start to the
eleventh magnitude. These will probably amount to about one million and
a quarter. For procuring both sets of plates, instruments were
constructed precisely similar to that of the MM. Henry, which is a
photographic refractor, thirteen inches in aperture, and eleven feet
focus, attached to a guiding telescope of eleven inches aperture,
corrected, of course, for the visual rays. Each place covers an area of
four square degrees, and since the series must be duplicated to prevent
mistakes, about 22,000 plates will be needed for the Chart alone. The
task of preparing them was apportioned among eighteen observatories
scattered over the globe, from Mexico to Melbourne; but three in South
America having become disabled or inert, were replaced in 1900 by those
at Cordoba, Montevideo, and Perth, Western Australia. Meanwhile, the
publication of results has begun, and is likely to continue for at least
a quarter of a century. The first volume of measures from the Potsdam
Catalogue-plates was issued in 1899, and its successors, if on the same
scale, must number nearly 400. Moreover, ninety-six heliogravure
enlargements from the Paris Chart-plates, distributed in the same year,
supplied a basis for the calculation that the entire Atlas of the sky,
composed of similar sheets, will form a pile thirty feet high and two
tons in weight![1576] It will, however, possess an incalc