1850-60] ROYAL ASTRONOMICAL SOCIETY 113 planets. Another successful exponent of the new art was Mr. Hartnup, of the Liverpool Observatory, who in 1854 obtained pictures of the moon said " to have outstripped all other attempts made elsewhere." He himself was disappointed with his results, a disappointment not unnatural when we realise that his confidence in the sharpness of image and minuteness of detail in his negatives led him to demand that they should stand enlargement from 1-3 inches to 50 feet, equivalent to 460 diameters. A modern dry plate at such an enlargement would show little more than the grain of the silver deposit, and it is no small tribute to the excellence of these early wet plates that any picture at all remained. In 1854 it was decided by the Royal Society that a photo- heliograph should be established at Kew for the purpose of making a daily record of the state of the solar surface. De la Rue undertook the supervision of the design and erection of the instrument, and it was put into permanent use in 1858 March, remaining at Kew until its transfer to the Royal Observatory, Greenwich, in 1872. In that same year (1858) the whole astronomical world was thrilled by the appearance of a great comet (Donati's), which for many weeks presented a spectacle of extraordinary beauty and interest. Attempts were made to photograph it, but there was then no instrument provided with the guiding accessories necessary for keeping the image fixed on the plate during a prolonged exposure. De la Rue could obtain nothing in sixty seconds, and the only recorded photograph is one taken by Mr. Usherwood on Walton Common with a stationary camera furnished with a portrait lens of short focus. We can only regret that no possessor of an equatoreal thought of the device of strapping such a camera on to his telescope, which would have given with a short exposure a record of this unique celestial object. We must content ourselves with noting the fact that Mr. Usherwood's was the first photograph taken of a comet. It would not be right to leave this subject of photography without some allusion, however brief, to the work which was being done at the other side of the Atlantic. Two names are prominent, G. P. Bond at Harvard (1851) and L. M. Rutherfurd from 1864. In both cases their main object was the same, to develop photo- graphy as a method of precision for the delineation and recording of star positions ; to what splendid development this has attained at Harvard is well known to all and need not be described here. Concurrently with this advance in celestial photography, a great improvement was made in the reflecting telescope by the substitution of the silver-on-glass for the speculum metal mirror. The previous decade had seen the completion of Lord Rosse's 6-foot telescope and its application to the resolution of nebulae, a 8
