ON PHOTOGRAPHIC PROCESSES. 
3 
obtained even an approach to it. A paper prepared (Oct. 28, 1842) according- to the 
instructions of Art. 229 in every respect, except in the proportion of tartaric acid 
(which was somewhat less than that recommended), proved very little sensitive. A 
strip of this paper, half shaded, acquired after a few minutes’ exposure to sunshine 
only a feeble brown colour over the sunned portion. Being then withdrawn, it was 
washed over with nitrate of mercury. Immediately the sunned portion began to 
darken very rapidly while the shaded part was unaffected, and ultimately assumed a 
deep brown hue. Exposed while yet wet to the sunshine, this passed rapidly to in- 
tense blackness, while the portion originally shaded, which had undergone the same 
subsequent application, and which was now equally exposed to the sun, sustained in 
the short time required for bringing on this effect, no appreciable change. Indeed 
it seemed rather to have become more insensible than before. 
235. Not alone nitrate of mercury is capable of thus exciting or stimulating the 
dormant photographic impression on such paper. To my very great surprise, I found 
the same effect to be produced by water sparingly applied, so as only to moisten 
the paper. Across the sunned and shaded portions of a strip of the mercurialized 
paper, exposed till a pale brown was developed in the former portion, were drawn two 
streaks, one of weak nitrate of mercury and one of spring water. Both, after a very 
short interval, passed to an intense brown on the sunned half, the shaded remaining 
unchanged. Edging the streak produced by the nitrate was a black border, that 
produced by the water was uniform. The whole paper was now exposed for a short 
time to the sun, which rapidly converted to intense blackness both the streaks on the 
previously sunned half, while it produced no perceptible change in the other. I found 
this experiment to succeed on many different varieties of paper, and with very consi- 
derable latitude in the dosage of the ingredients. It was most successful in the case 
of a paper prepared with a cream, formed by mixing one measure amxnomo-tartrate 
of iron (strength i\ # ) and two saturated protonitrate of mercury, leaving out the 
free tartaric acid altogether, which, among many other doses of these two ingre- 
dients, proved also, generally, the most sensitive to light. 
236. Led by these indications I prepared a paper by washing, first with a weak 
solution of ammonio-citrate of iron (strength -^g-), and when dry, with saturated pro- 
tonitrate of mercury. It was exposed when barely dry enough, not to feel damp, with 
an engraving in a frame to a hazy and declining sun. In about twenty minutes a 
very pale and feeble photograph was produced. Excited as above, by water, it 
gained but little in intensity (for it deserves remark that the increase of apparent in- 
tensity produced by either water or the nitrate, is in direct proportion to the force of 
the original impression, which, as observed, was in this case very faint). It was then 
held for about five minutes in the sun (near setting), and by degrees, and with the 
utmost regularity of gradation over every part of the picture, each line assumed an 
* By this I understand one part (by weight) salt + 11 water. 
B 2 
