[.1h95.] 
XXI. Proceedings of Learned Societies. 
ROYAL SOCIETY, 
[Continued from vol. xxi. p. 228.] 
Nov 17, HE following papers were read, viz.— 
1842. 1. Postscript to a paper “ On the Action of the Rays 
of the Solar Spectrum on Vegetable Colours.” By Sir John Frede- 
rick William Herschel, Bart., F.R.S., &e. 
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 
Chrysotype, 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 
of sharpness, and singular beauty and delicacy of tint. From his 
further researches on this subject he deduces the following conclu- 
sions: first, that it is the heat of the rays, not their light, which ope- 
rates the change; secondly, that this heat possesses a peculiar che- 
mical 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 obscurely hot iron abounds especially in 
rays analogous to thuse of the region of the spectrum above de- 
scribed. 
The author then describes the photographic properties he has 
discovered to belong to mercury, a metal which he finds to possess, 
in an eminent degree, direct photographic susceptibility*. 
2. “Observations de la variation de la déclinaison et intensité hori- 
zontale magnétiques observées 4 Milan pendant vingt-quatre heures 
consécutives, le 22 et 23 Juin, le 20 et 21 Juillet, le 26 et 27 d’Aout, 
le 21 et 22 Septembre, et le 19 et 20 Octobre, 1842,” rapportées 
par Robert Strambrecchi, premier éléve adjoint. 
* This paper will appear entire in a future Number of the Philosophical 
Magazine.—Eprr. 
