1869

From the Report to the Board of Visitors it appears that application had been made for an extension of the grounds of the Observatory to a distance of 100 feet south of the Magnetic Ground, and that a Warrant for the annexation of this space was signed on 1868, Dec. 8.  The new Depot for the Printed Productions of the Observatory had been transferred to its position in the new ground, and the foundations for the Great Shed were completed.—­“The courses of our wires for the registration of spontaneous terrestrial galvanic currents have been entirely changed.  The lines to Croydon and Deptford are abandoned; and for these are substituted, a line from Angerstein Wharf to Lady Well Station, and a line from North Kent Junction to Morden College Tunnel.  At each of these points the communication with Earth is made by a copper plate 2 feet square.  The straight line connecting the extreme points of the first station intersects that connecting the two points of the second station, nearly at right angles, and at little distance from the Observatory.—­The question of dependence of the measurable amount of sidereal aberration upon the thickness of glass or other transparent material in the telescope (a question which involves, theoretically, one of the most delicate points in the Undulatory Theory of Light) has lately been agitated on the Continent with much earnestness.  I have calculated the curvatures of the lenses of crown and flint glass (the flint being exterior) for correcting spherical and chromatic aberration in a telescope whose tube is filled with water, and have instructed Mr Simms to proceed with the preparation of an instrument carrying such a telescope.  I have not finally decided whether to rely on Zenith-distances of gamma Draconis or on right-ascensions of Polaris.  In any form the experiment will probably be troublesome.—­The transit of Mercury on 1868, Nov. 4th, was observed by six observers.  The atmospheric conditions were favourable; and the singular appearances usually presented in a planetary transit were well seen.—­Mr Stone has attached to the South-East Equatoreal a thermo-multiplier, with the view of examining whether heat radiating from the principal stars can be made sensible in our instruments.  The results hitherto obtained are encouraging, but they shew clearly that it is vain to attempt this enquiry except in the most superb weather; and there has not been a night deserving that epithet for some months past.—­The preparations for observing the Transits of Venus were now begun in earnest.  I had come to the conclusion, that after every reliance was placed on foreign and colonial observatories, it would be necessary for the British Government to undertake the equipment of five or six temporary stations.  On Feb. 15th I sent a pamphlet on the subject to Mr Childers (First Lord of Admiralty), and in April I wrote to the Secretary, asking authority for the purchase of instruments.  On June 22nd authority is given to me for the instruments: