17 de Novembro
ABSTRACTS OF THE PAPERS,
PRINTED IN THE PHILOSOPHICAL TRANSACTIONS
ROYAL SOCIETY OF LONDON
Pag. 397, 398
Proceedings of the royal society
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 interestirig photographic processes dependent on those properties, which the favourable weather of the summer has enabled to discover. The author also describes a better method of fixing the picture, in the process which he has denominated the Chrysotype, than which he had specified in the latter part of his paper. In this new method the hydriodate is substitued 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 particulary noticed, namely, that of simple passing over the ammonio-citrated paper, on which a latent picture has been impressed, 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, wich, at certain moment, possesses a high degree of sharpness, and singular beauty and delicacy of tint. From his further researches on this subject he deduces the following conclusions: first, that it is the heat of the rays, not their light, which operates the change; secondly, that this heat possesses a peculiar chemical quality, which is not possessed by the purely calorific rays outside of the visible spectrum, though far more intense; and thirdly, that the heat radiated from obscuraly hot iron abounds especcially in rays analogous to these of the region of the spectrum above described.
The author then describes the photographic +roperties he has discovered to belong to mercury, a metal which he finds to possess, in a eminent degree, direct photographic susceptibility.
A letter was also read from Sir John F. W. Herschel on the subject of Photography, adressed to S. Hunter Christie, Esq., Sec. R. S.