1880-1920] ROYAL ASTRONOMICAL SOCIETY 213 in this case the change came somewhat more gradually. This is not the place to describe the development of the use of photo- graphy either in delineating the features of sun, moon, nebulae, and their spectra, or in providing new methods of determining stellar positions, which until the advent of the gelatine-emulsion process about the middle of the seventies made very slow progress. The immense increase in sensitiveness of the photographic plate then obtainable almost at once changed astronomical photography from a curious toy into a most important adjunct to an observatory. Draper led the way by photographing the nebula in Orion in 1881, but his early death in the following year left the field open to Common, whose brilliant success was, in 1884, rewarded by the bestowal of the Society's Gold Medal. In 1883, Common proposed to Gould, who had photographed about seventy star-clusters at Cordoba, a joint arrangement for photographing the whole heavens.* Gould's work in South America was so near its close that he was unable to undertake anything new, and the immense labour of measuring the plates would in any case have tended to deter him. But by that time the problem of charting the stars by photography had attracted attention in various quarters. The great number of stars visible on the Cape photographs of the great comet of 1882 showed the possibilities of the new method, and so did the experiments made in 1884 by the brothers Henry at Paris. They found in the course of their continuation of Chacornac's Ecliptic Atlas, that their task became impossible when they approached the Milky Way, so that they were compelled to try the use of photography. In 1885 June, Admiral Mouchez, Director of the Paris Observatory, sent to our Society a cliche* of part of the Milky Way as well as an enlarged photograph of the same. The instrument employed had a specially constructed object-glass of 34 cm. aperture, made by Paul and Prosper Henry, and three exposures of an hour each were made, producing three images of every star, 4 "-5 apart, to guard against accidental spots on the plate being mistaken for stars. In the covering letter f it was suggested that six or eight observatories ought to combine in order to produce in the course of eight or ten years a complete set of maps of the whole heavens. Quite independently of the work done at Paris, Gill had in the meantime started work at the Cape, to continue by photography Argelander's and Schonfeld's Durchmusterung to the South Pole. The first plates were taken on 1885 April 2, and the work was completed by the end of 1890. Thanks to the devoted co-opera-
- The Observatory, 9, 326.
t Monthly Notices, 46, i. Compare Mouchez' account, Comptes Rendus, 1885 May ii.
