4 
SIR J. F. W. HERSCHEL ON CERTAIN IMPROVEMENTS 
inky blackness, the lights and shades being exquisitely preserved in their due pro- 
portions, and the ground being hardly perceptibly discoloured. The result was a 
very beautiful and perfect negative photograph. 
237. This singular power of water to excite the dormant impression, strongly 
recalls the analogous power of moisture to deepen the tints photographically im- 
pressed on auriferous papers, of which an instance is given in Art. 45, and of which 
a still more striking example is shown as follows. Let a paper be washed first with 
ammonio-citrate of iron, and when dry with neutralized chloride of gold, and 
thoroughly dried in the dark. It is then, apparently, almost insensible to light ; a 
slip of it half exposed to sun being hardly impressed in any perceptible degree in 
many minutes ; yet if breathed on, the impression conies out very strong and full, 
deepening by degrees to an extraordinary strength. Treated in the same manner, 
silver also exhibits a similar property*. Nor, indeed, is there any feature in photo- 
graphy more general or more remarkable than the influence exercised by the presence 
of a certain degree of moisture in favouring the action of light, whether direct or 
indirect. 
238. There is this difference, however, in the excitement produced by simple water 
and by the mercurial solution, viz. that the latter is permanent, the former liable to 
fade ; at least I have found this to be the case with the brown tinge produced by it 
in shade, though when blackened by a second exposure to sun no difference is per- 
ceived, On the other hand, when the nitrate is used, the brown hue frequently passes 
to absolute blackness without any subsequent exposure to sunshine ; and in that case 
the photographs produced have an intensity and opacity scarcely, if at all, inferior to 
that of printing ink. 
239. This high degree of opacity and depth, together with the comparative insen- 
sibility of the ground, is evidently capable of being most usefully applied to the pro- 
duction of retransfers. In fact, the photographs so produced being negative are so 
far fitted for the purpose, and if used as models while in this, their transition state, 
and as it were self-fixed, so far from being injured by the transmission of light, they 
are actually acquiring additional sharpness and depth by every beam which passes. 
By seizing therefore the right point of dryness, and by using a very sensitive paper to 
receive the impression, there is no reason to doubt of success in procuring very per- 
fect positive transfers. Some trials I have made have satisfied me as to the practi- 
* Note added Dec. 21. — The excitement is produced on such paper by the ordinary moisture of the atmo- 
sphere, and goes on slowly working its effect in the dark, apparently without other limit than is afforded by 
the supply of ingredients present. In the case of silver, it ultimately produced a perfect silvering of all the 
sunned portions. Very singular and beautiful photographs having much resemblance to Daguerreotype pictures, 
are thus produced ; the negative character changing by keeping, and by quite insensible gradations, to positive ; 
and the shades exhibiting a most singular chatoyant change of colour from ruddy-brown to black when held 
more or less obliquely. No doubt also gold pictures with the metallic lustre might be obtained by the same 
process, though I have not tried the experiment.— J. F. W. H. 
