The image of the sun in the principal focus of this telescope is about six-tenths of an inch in diameter; but a magnifying lens is used, so that the photograph actually obtained is about eight inches. Even with this great enlargement, the light of the sun is so intense that with the slowest photographic plates that are made the exposure has to be for only a very small fraction of a second. This is managed by arranging a very narrow slit in a strip of brass. The strip is made to run in a groove across the principal focus. Before the exposure, it is fastened up so as to cut off all light from entering the camera part of the telescope. When all is ready, it is released and drawn down very rapidly by a powerful spring, and the slit, flying across the image of the sun, gives exposure to the plate for a very minute fraction of a second--in midsummer for less than a thousandth of a second. Two of these photographs are taken every fine day at Greenwich; occasionally more, if anything specially interesting appears to be going on. But in our cloudy climate at least one day in three gives no good opportunity for taking photographs of the sun, and in the winter time long weeks may pass without a chance. The present Astronomer-Royal, Mr. Christie, has therefore arranged that photographs with precisely similar instruments should be taken in India and in the Mauritius, and these are sent over to Greenwich as they are required, to fill up the gaps in the Greenwich series. We have therefore at Greenwich, from one source or another, practically a daily record of the state of the sun's surface. More recently the 'Dallmeyer' photo-heliograph, though still retained for occasional use, has been superseded generally by the 'Thompson'; a photographic refractor of nine inches aperture, and nearly nine feet focal length, presented to the Observatory by Sir Henry Thompson. The image of the sun obtained after enlargement in the telescope, with this instrument, is seven and a half inches in diameter. The 'Thompson' is mounted below the great twenty-six-inch photographic refractor,--also presented to the Observatory by Sir Henry Thompson,--in the dome which crowns the centre of the New Observatory.