154 Dr Fyfe on Photography. 



is instantly converted to yellow, provided the solution is of 

 sufficient strength ; if weak, the action goes on slowly. In 

 some impressions which I had attempted to preserve in this 

 way, I observed, that, when exposed to light, they began to 

 fade, which induced me to try the effect of light on darkened 

 paper, soaked in solution of iodide, of such strength, that it 

 just failed to attack it instantly. In my first attempt I suc- 

 ceeded in bleaching the paper, but in my next I failed. On 

 considering the circumstances under which these trials were 

 made, I found that the only difference between them was, 

 that in the first the paper was moist, in the last it was dry. 

 Accordingly, on repeating the experiment with the paper 

 moist, I again succeeded in getting a delineation of the object 

 placed on the paper, as distinct and altogether as brilliant as 

 those obtained by the other process. 



The method which I now follow is, after preparing the phos- 

 phate paper, to darken it, then immerse it in solution of iodide 

 of potassium, of such strength that it does not act instanta- 

 neously, and, when still moist, to expose it to light with the 

 object on it, and continue the exposure 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 would go on slowly, but is hastened by the light ; of 

 course, if the object on the paper is impervious to light, the 

 impression is black throughout, but if it is of different density, 

 so as to allow the light to be differently transmitted, the im- 

 pression presents the lights and shades as in the object itself; 

 because those places behind the dense pieces retain their ori- 

 ginal blackness, while those behind the less dense are more or 

 less bleached, just according to the transmission of the light. 

 When impressions thus procured are kept, they begin to fade, 

 owing to the slow but continued action of the iodide of potas- 

 sium ; hence the necessity of a preservative process. After 

 repeated trials, I Jiave 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 not acted 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 



