H2 HISTORY OF THE [1850-60 theory of light and taking the place that he held so long of being the ultimate court of appeal on subjects that were sufficiently difficult to baffle all other enquirers. Just by way of a date point, we may note that the Change of Refrangibility of Light was read to the Royal Society in two parts, 1852 May and 1853 June respectively. Turning to astronomy, we find that a change no less fundamental was coming, and though we should, perhaps, not be justified in claiming for the decade under review its actual arrival, we may confidently assert that, looking back now on these ten years, we see in them a period of great development, containing promise of changes even more profound. Anyhow, it is clear that the outlook and aspirations of the astronomer of 1860 were of a different nature from those of his predecessor of 1850. With the invention of photography and the discovery of spectrum analysis, the astronomer's powers were multiplied and the whole scope and possibilities of his science enormously enlarged. Up to 1850 the only photographic process known was the daguerreotype, a method producing pictures of exquisite fineness of detail but demanding high technical skill, and moreover requiring exposures of such length that it was useless as an accessory to the telescope except for the sun. The first recorded astronomical photograph is one of the total solar eclipse of 1851 July 28, for which see volume 41 of the Memoirs, Royal Astronomical Society. It shows the corona extending from the limb for about one-fifth of the diameter of the moon. In 1850 the collodion wet plate was invented, and a photographic method with a not too difficult technique and requiring exposures of about one-thirtieth of those previously necessary was thus placed in the hands of experimenters. Sir David Brewster, in his Presidential address to the Edinburgh meeting of the British Association held in that year, devoted a large part of his time to the subject of astronomy, and said, " Though but slightly connected with astro- nomy, I cannot omit calling your attention to the great improve- ments, I may call them discoveries, which have been recently made in photography." This view, that the connection between astronomy and photography was slight, was, however, not shared by others, who saw in the new science a most promising addition to the astronomer's tools, and lost no time in attempting to bring it into service. An early and successful experimenter was Warren de la Rue, an esteemed Fellow of the Society who obtained the Gold Medal in 1862 and became President in 1864. He devoted himself with great energy and at considerable outlay to the construction of a new telescope specially designed for this work, and secured numberless very beautiful photographs of the moon, also some of stars and
