nomical Society in 1841 (Memoirs, xii. 201), and made his final report, recommending less stringent, but more practicable measures, to the British Association in 1844 (Report, p. 32).
Herschel discovered in 1840 the variability of α Orionis (Memoirs Roy. Astr. Soc. xi. 269), and was, on 17 March 1843, among the first observers in England of the great comet (Proceedings Roy. Soc. iv. 450). In a ‘Note on the Art of Photography’ he had explained before the Royal Society, on 14 March 1839, his independent invention of the photographic use of sensitised paper (ib. iv. 131); and an essay ‘On the Chemical Action of the Rays of the Solar Spectrum on Preparations of Silver and other Substances,’ read on 5 March 1840 (Phil. Trans. cxxx. 1), obtained the third royal medal bestowed upon him by that body. It announced the use as a fixing agent of hyposulphite of soda, the solvent power of which upon the salts of silver he had discovered in 1819 (Edinb. Phil. Journal, i. 8); it originated the application to photographic prints of the terms ‘positive’ and ‘negative,’ adverted to ‘lavender grey’ rays beyond the violet, and described experiments on the ‘chemical analysis of the solar spectrum,’ by which an important new field was thrown open to research. The apparatus employed in them formed part of the Loan Collection of Scientific Instruments at South Kensington in 1876. His efforts to obtain coloured photographs were only partially successful; but his reproduction in 1843 of an engraving of the Slough forty-foot reflector was the first example of a photograph on glass (Abney, Treatise on Photography, p. 5). His discovery in 1845 (Phil. Trans. cxxxv. 147) of the ‘epipolic dispersion’ of light produced by sulphate of quinine and some other substances, led the way to Sir George Stokes's explanation of the phenomena of fluorescence.
By the end of 1842 he had performed without assistance the computations necessary for the publication of his Cape observations. In September 1843 the letterpress was ‘fairly begun,’ and after some delays the work appeared in 1847, at the cost of the Duke of Northumberland, in a large quarto volume, entitled ‘Results of Astronomical Observations made during the years 1834–8 at the Cape of Good Hope.’ Besides the catalogues of nebulæ and double stars, it included profound discussions of various astronomical topics, and was enriched with over sixty exquisite engravings. He insisted in it upon the connection of sun-spots with the sun's rotation, and started the ‘cyclonic theory’ of their origin. He investigated graphically the distribution of nebulæ, but fluctuated in his views as to their nature. Regarding them in 1825 as probably composed of ‘a self-luminous or phosphorescent substance, gradually subsiding into stars and sidereal systems’ (Memoirs Royal Astronomical Society, ii. 487), he ascribed to them later a stellar constitution, and finally inclined to suppose them formed of ‘discrete luminous bodies floating in a non-luminous medium’ (Results, &c. p. 139). Herschel stands almost alone in his attempt to grapple with the dynamical problems presented by star-clusters, and his analysis of the Magellanic Clouds was decisive as to the status of nebulæ. For these labours he received the Copley medal in 1847, and a special testimonial from the Royal Astronomical Society in 1848.
In April 1840 Herschel removed from Slough to a more commodious residence, named Collingwood, at Hawkhurst in Kent, and in December 1850 accepted the post of master of the mint, on its conversion from a ministerial into a permanent office. The reorganisation of the establishment devolved upon him, and the duties connected with it were rendered the more uncongenial by the partial separation from his family which their fulfilment required. He was one of the jury for scientific instruments at the Great Exhibition, and a member of the royal commission appointed in 1850 to inquire into the course of study at the universities of Oxford and Cambridge. His health suffered, and his resignation of his position at the mint was unwillingly accepted in 1855.
Herschel afterwards led a retired life at Collingwood. The collection and revisal of his father's and his own labours was an arduous task, partially completed by the presentation to the Royal Society on 16 Oct. 1863 of a ‘Catalogue of 5,079 Nebulæ and Clusters’ (all then known), reduced to the common epoch 1860 (Phil. Trans. cliv. 1). He next undertook the amalgamation into a catalogue of his father's ‘classes’ of double stars, and on 14 Dec. 1866 read before the Royal Astronomical Society a ‘Synopsis of all Sir William Herschel's Micrometrical Measures of the Double Stars described by him’ (Memoirs, xxxv. 21). The autograph observations of the 812 pairs catalogued accompanied the paper, and are deposited in the library of the society. Herschel's general and descriptive catalogue of double stars was his last great undertaking. He finished before his death the arrangement in right ascension of 10,320 composite objects, with the synoptical history of two-fifths of them; and from his papers bequeathed to the Royal Astronomical Society the incomplete cata-
