The present year would be memorable in astronomical history for the
practical beginning of the photographic chart and catalogue of the
heavens which took their origin in an international conference which
met in Paris in 1887. The decisions of the conference in their final
form provided for the construction of a great chart with exposures
corresponding to forty minutes' exposure at Paris, which it was
expected would reach down to stars of about the fourteenth magnitude.
As each plate was to be limited to four square degrees, and as each
star, to avoid possible errors, was to appear on two plates, over
22,000 photographs would be required. A second set of plates for a
catalogue was to be taken, with a shorter exposure, which would give
stars to the eleventh magnitude only. The plans were to be pushed on
as actively a possible, though as far as might be practicable plates
for the chart were to be taken concurrently. Photographing the plates
for the catalogue was but the first step in this work, and only
supplied the data for the elaborate measurements which would have to
be made, which were, however, less laborious than would be required
for a similar catalogue without the aid of photography.