OF THE SOLAR SPECTRUM ON VEGETABLE COLOURS. 
209 
piousness as to terminate the spectrum by a well-defined semicircle e, fig. 12, No. 3, 
and to give to the whole portion c e the appearance of a brilliant and purely blue 
spot. Finally, after long-continued action, the interior browned oval above-men- 
tioned was found to have been prolonged into a figure of the form No. 4, fig. 12 (di- 
stinctly seen at the back of the paper), of which the termination by a narrow neck 
and circular enlargement indicates the definite action of a ray much further removed 
along the axis of the spectrum. Washing with water at once obliterates this part of 
the phenomenon, destroys the brown colour, and leaves simply the bleached cometic 
train, in singularly striking contrast with the dark ground of the paper. Specimens 
of the spectrum itself are subjoined for inspection. 
216. The black positive paper used in the above experiment (which has been often 
repeated with the same results) contains free nitrate of silver. If this be washed out, 
the darkening at the lower end of the spectrum is not produced, but in its place the 
feeble subsequent bleaching in the region above-mentioned commences at once. And 
if besides washing with mere water, the paper be subsequently washed with a neutral 
hyposulphite to remove all chloride of silver, it is reduced to a state of perfect insen- 
sibility. It is therefore to this latter element that the direct action of the bleaching 
rays is to be referred. A few months’ keeping also destroys the positive sensibility 
of the paper in question entirely. 
J. F. W. Herschel. 
Collingivood, 
June 13, 1842. 
Postscript added August 29, 1842. 
217. I gladly avail myself of the permission accorded by the President and Council 
to append to this communication, in the form of a Postscript, some additional facts 
illustrative of the singular properties of iron as a photographic ingredient, which have 
been partially developed in the latter articles of it, as well as an account of some 
highly interesting photographic processes dependent on those properties, which the 
superb weather we have lately enjoyed has enabled me to discover, 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 hydrobroinate 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 contact with other papers. 
It is as follows : — As soon as the picture is satisfactorily brought out by the auriferous 
liquid (Art. 212.) it is to be rinsed in spring water, which must be three times re- 
newed, 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 
2 E 
MDCCCXLII. 
