Page:History of the Royal Astronomical Society (1923).djvu/184

 156 HISTORY OF THE [1860-70 the construction and regular use of which for recording the state of the surface of the sun had been repeatedly advocated by Sir John Herschel, was installed in the dome on the top of the Obser- vatory building under the direction of De la Rue. A couple of years sufficed to overcome most of the difficulties of the novel work, though the illness and death of John Welsh, Superintendent of the Observatory, delayed progress. Welsh was succeeded by Balfour Stewart, and then began that scientific partnership between De la Rue and Stewart which is made memorable by their joint communications published in the Philosophical Transactions of 1869 and 1870. In 1860 it was decided that the photoheliograph should be put at De la Rue's disposal for the total eclipse of the sun of 1860 July 18 ; and the occasion was made memorable by the successful photographic operations ; for they served to prove conclusively that the brilliant extrusions seen round the dark limb of the moon were indeed prominences connected with the sun, and, like the sun itself, subject to gradual eclipse by the moving moon. When De la Rue had completed the reduction of the observations with the help of a special micrometer devised by himself, he found himself impelled, by reason of the heavy weight of magnetic and meteorological work undertaken at Kew, to undertake the daily solar observations at Cranford. The optical tube of the photo- heliograph was removed thither at the beginning of 1862, and was attached to the equatorial, where it remained at work until 1863 February. The instrument was then re-erected in the dome at Kew, and continued there in active operation for ten years more. In 1873 February it was moved to the Royal Observatory, Greenwich, and the fine series of solar records has been continued there with frequent improvement in the instrumental equipment and in the contributions of photographic records sent from observing stations in various parts of the globe. It is interesting also to note the part played by Carrington's work in furthering the initial aims of the research. In the preface of his " Observations of Sun-spots " (1863), Carrington recounts how, when he set up his observatory at Red Hill, in the summer of 1852, for meridian observations of circumpolar stars, he was led to examine a series of drawings of the sun's disk in the possession of the Society. The discovery of the similarity in phase of the periodic recurrence of sun-spots and of magnetic disturbances had served to enhance the value of Schwabe's observations of the spots. Carrington had hopes of deriving, from observations extending over eleven years, the means of tracing system in the distribution and possible movements of spots and of detecting the true period of rotation. Even after Sir John Herschel, in 1854, had recommended