334 
MR. ROBERT HUNT ON THE INFLUENCE OF IODINE. 
except the lights of the picture. To explain the rationale of this definite arrangement 
of a dense precipitate, is beyond my power. 
32. Sir John Herschel has recently shown that corrosive sublimate obliterates 
the ordinary photographic drawings, and that they may be restored by the hyposul- 
phite of soda. An equally singular effect is produced upon some of the papers above- 
mentioned. If the ordinary muriated photographic paper darkened (7. 8.) be used to 
procure a drawing by the process which occupies our attention, on being treated with 
a solution of the bichloride of mercury, the dark parts of the picture are whitened, 
and it of course becomes of the negative variety ; but it is speedily rendered positive, 
or the lights and shades corrected, by the hyposulphite of soda. We have it thus in 
our power to produce upon the same sheet two very marked varieties of photographic 
drawings. 
Devonport, May 5, 1840. 
Postscript. 
(a.) I find the invisible photographic image, on the papers prepared by sulphura- 
tion, becomes evident without the aid of mercurial vapour, by simply soaking for 
some time in a solution of corrosive sublimate. The picture thus formed is extremely 
faint, but the fact is worthy of notice, as it may ultimately lead to the discovery of a 
process, by which the disengagement of the light-created picture may be effected by 
more simple means than at present. 
( b .) If papers saturated with starch prior to being washed with the salt of silver 
and exposure to the sulphuretted hydrogen gas, the silver being applied on one side 
only, be, when completed, placed between the leaves of a printed book and subjected 
to some little pressure, the printing is faithfully copied off on the unsilver'ed side. It 
is with difficulty the letters can be made out when the paper is taken from the vo- 
lume, but by immersing it in a solution of iodine they become a deep blue, verging 
on a black, whilst the ground remains of a yellow colour, which by the continued 
action of the fluid changes to a light violet. On drying, the ground becomes con- 
siderably darker, and the printing, though evident, indistinct, appearing in some posi- 
tions the lightest, in others the darkest portions of the paper. These, and the facts 
above-mentioned, render it probable that we may eventually succeed in copying 
prints, &c. by mere juxtaposition. 
R. H. 
May 19, 1840. 
