422 Proceedings of the Society of Arts. 



drew Fyfe, M.D., F.R.S.E., Vice-Pres. Soc. Arts. (641.) Specimens 



were exhibited. 



Before proceeding to describe his method of taking impressions -with- 

 out having a reverse, Dr Fyfe stated that he had received communications 

 from several gentlemen, mentioning that they had repeated his experi- 

 ments with the phosphate of silver paper, and also with the ammonia, 

 as a preservative. With regard to the latter, it was found by them all to 

 prevent the farther action of the light on the specimens, provided it was 

 properly applied. As a test of its proper application, he stated, that the 

 best method is to put the paper into a diluted solution of the ammonia, 

 and to leave it for a short time, till all the yellow parts of the impression 

 become white, shewing that the whole of the yellow phosphate is washed 

 out. An impression on paper, taken by Mr Wm. Forrester, lithographer, 

 was shewn. In this the lights and shadows were preserved as in the ori- 

 ginal, by covering the light parts of the drawing from which it was taken 

 with a dark ground, and leaving the darker parts lighter and lighter, so 

 as to allow the greater transmission of light through what, in the original, 

 was the darkest. A lithographic stone was likewise shewn, on which an 

 Impression was taken by Mr Nichol, lithographer, by covering the stone 

 with phosphate of silver, and then, after putting an engraving on it, 

 exposing it to light in the usual way. Dr Fyfe then proceeded to de- 

 scribe the process by which he had succeeded in getting impressions, in 

 which the lights and shadows are not reversed. For this purpose the 

 phosphate paper is first darkened by the action of light ; it is then im- 

 mersed in a solution of the iodid of potassium, and, while still moist, ex- 

 posed to light, with the object, the impression of wliich is to be taken, 

 placed on it, and left till the whole of the paper exposed becomes yel- 

 low, and when removed it exhibits a distinct representation of the ob- 

 ject. In this process there is a tendency of the iodid to convert 

 the dark phosphate to yellow iodid of silver, which it does instantly 

 when the solution is strong, but very slowly when it is weak, unless it 

 is exposed to light, and then the action goes on rapidly. It was ob- 

 serving this that induced Dr Fyfe to try the influence of light on phos- 

 phate paper besmeared with iodid of potass, by which he was led 

 to the discovery. Of course, when an object, which allows the light 

 to pass through it differently, is put on the paper, those parts on which 

 the denser portions of the object are placed still retain their dark colour, 

 the other parts are tinged, just according to the transmission of the light. 

 When impressions thus prepared are kept, they gradually begin to fade, 

 owing to the continued action of the iodid of potass, and hence the ne- 

 cessity of submitting them to a preservative process. After numerous 

 trials, that which seemed to answer best, was merely immersing them in 

 water for a few minutes, and in some cases even allowing a stream of 

 water to flow gently on them, so as to wash out the whole of the iodid 

 of potass not acted on — in this way the agent which tends to decolorize 

 the blackened phosphate, seems to be removed. Several specimens of 



