A PHOTOGRAPHIC CHART.
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.