Turning back to the transit room, and leaving it by the south-west door, we come into the little passage which leads at the back of Bradley's transit room into the lower computing room. Just inside this passage, on the left-hand side, there is a little room of a most curious shape, the 'reflex zenith room.' Here is fixed a telescope pointing straight upwards, the eye-piece being fixed by the side of the object-glass. The light from a star--the star Gamma Draconis--which passes exactly over the zenith of Greenwich, enters the object-glass, passes downwards to a basin of mercury, and is reflected upwards from the surface of the mercury to a little prism placed over the centre of the object-glass, from which it is reflected again into the eye-piece. By means of this telescope the distance of the star Gamma Draconis from the zenith could be measured very exactly, and, consequently, the changes in the apparent position of the star due to aberration, parallax, and other causes could be very exactly followed, and the corrections to be applied on account of these causes precisely determined. This particular telescope was devised by Airy, and the observations with it were continued to the end of his reign. The germ of the idea may be traced back, however, to the time of Flamsteed, who would seem to have occasionally observed Gamma Draconis from the bottom of a deep well; the precise position of the well is not, however, now known. Later, Bradley set up his celebrated 12-1/2-foot zenith sector, still preserved in the transit room, first at Wanstead and then at Greenwich, for the determination of the amount of aberration. Later, a zenith tube by Troughton, of 25 feet focus, was used by Pond in conjunction with the mural circle for observations of Gamma Draconis in order to determine the zenith point of the latter instrument. These telescopes for special purposes have passed out of use. Observations with the spectroscope have been suspended for some years. The work of the Astrographic Department will come to an end, in the ordinary course of events, when the programme assigned to Greenwich in the International Scheme is completed. Within the last few years a new department has come into being at Greenwich--a department which has been steadily worked at many foreign public observatories, but only recently here. This is the Department of Double-Star Observation. The first double star, Zeta Ursæ Majoris, was discovered 250 years ago. Bradley discovered two exceedingly famous double stars whilst still a young man observing with his uncle at Wanstead--Gamma Virginis and Castor. Bradley made also other discoveries of double stars after his appointment to Greenwich, and Maskelyne succeeded him in the same line, but the great foundation of double-star astronomy was laid by Sir William Herschel.