[ ae Miscellanies. 183 
‘The method which I now follow is, after preparing the phosphate 
| paper, to darken it, then immerse it in solution of iodide of potassium, 
of such strength that it does not act instantaneously, and, when still 
moist, to expose it to light with the object on it, and continue the ex- 
posure till the exposed part of the paper becomes yellow. In this 
case, there is a tendency in the iodide to convert the dark phosphate 
to yellow iodide, which go on slowly, but is hastened by the light; of 
course, if the object on the paper is impervious to light, the impres- 
sion is black throughout, but if it is of different density, so as to allow 
the light to be differently transmitted, the impression presents the 
lights ‘and shades as in the object itself; because those places behind 
the dense pieces retain their original blackness, while those behind 
the less dense are more or less bleached, just according to the trans- 
Mission of the light. Whenimpressions thus procured are kept, they 
begin to fade, owing to the slow but continued action of the iodide of 
potassium; hence the necessity of a preservative process. After re- 
Peated trials, I have found, that by far the simplest and the best is 
_ merely immersion in water, so as to carry off the whole of the iodide — 
| of potassium notacted on by the phosphate, and by which any farther 
| action is completely prevented. By this method, the specimens do 
Not lose in the least their original beauty, and they may be exposed 
to continued sunshine without undergoing the slightest alteration. 
Thave succeeded also in taking impressions with the chloride in the 
same way—but it is necessary for the success of the process, to use 
the Solution of the iodide much weaker than for the phosphate, be- 
Cause the chloride is more easily acted on. In both cases it ought to 
’@ made of such strength that it just acts, and then, before using it, 
| it must be weakened by the addition of a little water. For the phos- 
+ phate, it will be found, in general, that 1 of salt to 10 of water, and 
or the chloride, that about 30 of water, will give a solution of the 
Tequisite strength. Of course, in preserving the specimens, the pre- 
Utions as to washing and pressure must be attended to. 
| HL. Pp erfection of the Art, as stated in Notes on Daguerre’s Pho- 
: tography. By Sir Joun Roptson.* 
Sir—Ip compliance with the request, that I should commit to wri- 
ling and put into your hands the substance of what I communicated 
‘0 the Society of Arts in reply to the questions put to me at the last 
Meeting, I beg to state, that circumstances having led to my be- 
ng included in a small party of English gentlemen who were lately in- 
gg Seatetary to Royal Society of Edinburgh, &c. &c. (Communicated by the 
ty of Arts.) Edinb. Jour. 
Ps 
