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 li-ght 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. "When impressions 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 olf the whole of the iodide 

 of potassium notacted on by the phosphate, and by Avhich 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. 



I have 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 

 be 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 

 for the chloride, that about 30 of water, will give a solution of the 

 requisite strength. Of course, in preserving the specimens, the pre- 

 cautions as to washing and pressure must be attended to. 



III. Perfection of the Art, as stated in Notes on Daguerre's Pho- 

 tography. By Sir John Robison.* 



Sir — In compliance with the request, that I should commit to wri- 

 ting and put into your hands the substance of what I communicated 

 to 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- 

 ing included in a small party of English gentlemen who were lately in- 



* Secretary to Royal Society of Edinburgh, &c. «fcc. (Communicated by the 

 Society of Arts.) Edinb. Jour. 



