PROCEEDINGS 



OF 



THE ROYAL SOCIETY. 



1842. No. 55. 



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 



