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XLI. On the Action of the Rays of the Solar Spectrum on 
Vegetable Colours, and on some new Photographic Processes. 
By Sir Joun F. W. Herscuet, Bart., K.H., FBS. 
[Continued from p. 180, and concluded. } 
Postscript added August 29, 1842. 
217. I GLADLY avail myself of the permission accorded by 
the President and Council to append to this communi- 
cation, in the form of a Postscript, some additional facts illus- 
trative of the singular properties of iron as a photographic in- 
gredient, which have been partially developed in the latter arti- 
cles of it, as well as an account of some highly interesting pho- 
tographic processes dependent on those properties, which the 
superb weather we have lately enjoyed has enabled me to dis- 
cover, as also to describe a better method of fixing the picture, 
in the process to which I have given the name of Chrysotype ; 
that described in Art. 212 proving insufficient. The new 
method (in which the hydriodate is substituted for the hydro- . 
bromate of potash) proves perfectly effectual ; pictures fixed 
by it not having suffered in the smallest degree, either from 
long exposure to sunshine, or from keeping; alone, or in con- 
tact with other papers. It is as follows :—As soon as the pic- 
ture is satisfactorily brought out by the auriferous liquid 
(Art. 212.) it is to be rinsed in spring water, which must be 
three times renewed, letting it remain in the third water five 
or ten minutes. It is then to be blotted off and dried, after 
which it is to be washed on both sides with a somewhat weak 
solution of hydriodate of potash. If there be any free chlo- 
ride of gold present in the pores of the paper, it will be dis- 
coloured, the lights passing to a ruddy brown; but they 
speedily whiten again spontaneously, or at all events, on throw- 
ing it (after lying a minute or two) into fresh water, in which, 
being again rinsed and dried, it is now perfectly fixed. 
218. If paper prepared as above recommended for the chry- 
sotype, either with the ammonio-citrate or ammonio-tartrate of 
iron, and impressed, as in that process, with a latent picture, 
be washed with nitrate of silver instead of a solution of gold, a 
very sharp and beautiful picture is developed, of great intensity. 
Its disclosure is not instantaneous ; a few moments elapse with- 
out apparent effect ; the dark shades are then first touched in, 
and by degrees the details appear, but much more slowly than 
inthe case of gold. In two or three minutes, however, the 
maximum of distinctness will not fail to be attained. ‘The 
