PROCEEDINGS
OF
THE ROYAL SOCIETY.
November 17, 1842.
The MARQUIS OF NORTHAMPTON, President, in the Chair.
The following gentlemen were, by ballot, elected Auditors of the Treasurer's Accounts, on the part of the Society : viz. Martin Barry, M.D., Henry James Brooke, Esq., Robert Brown; Esq., D.C.L., Rev. James Gumming, M.A., and John Thomas Graves, Esq., M.A.
James Scott Bowerbank, Esq., and Charles Tov/neley, Esq., were balloted for and duly elected Fellows of the Society.
The following papers were read, viz : —
1. Postscript to a paper " On the Action of the Rays of the Solar Spectrum on Vegetable Colours." By Sir John Frederick William Herschel, Bart, F.R.S., &c.
An account is here given of some additional facts illustrative of the singular properties of iron as a photographic ingredient, and also of some highly interesting photographic processes dependent on those properties, which the favourable weather of the summer has enabled him to discover. The author also describes a better method of fixing the picture, in the process which he has denominated the Chri/sofi/pe, than that which he had specified in the latter part of his paper. In this new method the hydriodate is substituted for the hydrobromate of potass ; and the author finds it perfectly effectual ; pictures fixed by it not having suffered in the smallest degree, either from long exposure to sunshine or from keeping.
He next considers the class of processes in which cyanogen, in its combinations with iron, performs a leading part, and in which the resulting pictures are blue ; processes which he designates by the generic term Cyanotype. Their varieties appear to be innumerable, but one is particularly noticed, namely, that of simply passing over the ammonio-citrated paper, on which a latent picture has been im- pressed, very sparingly and evenly, a wash of the solution of the common yellow ferrocyanate of potass. As soon as the liquid is applied the negative picture vanishes, and is replaced, by very slow degrees, by a positive one, of a violet-blue colour on a greenish- yellow ground, which, at a certain moment, possesses a high degree
