210 Mr. Talbot on Photogenic Draiaing. 



before the trial is made) that its sensibility is greatly diminish- 

 ed, and, in some cases, seems quite extinct. But if it is again 

 washed with a liberal quantity of the solution of silver, it be- 

 comes again sensible to light, and even more so than it was at 

 first. In this way, by alternately washing the paper with salt 

 and silver, and drying it between times, I have succeeded in 

 increasing its sensibility to the degree that is requisite for re- 

 ceiving the images of the camera obscura. 



In conducting this operation it will be found that the results 

 are sometimes more and sometimes less satisfactory in conse- 

 quence of small and accidental variations in the proportions 

 employed. It happens sometimes that the chloride of silver 

 is disposed to darken of itself, without any exposure to light : 

 this shows that the attempt to give it sensibility has been car- 

 ried too far. The object is, to ajiproach to this condition as 

 near as possible without reaching it ; so that the substance 

 may be in a state ready to yield to the slightest extraneous 

 force, such as the feeble impact of the violet rays when much 

 attenuated. Having therefore prepared a number of sheets 

 of paper with chemical proportions slightly different from one 

 another, let a piece be cut from each, and, having been duly 

 marked or numbered, let them be placed side by side in a very 

 weak diffused light for about a quarter of an hour. Then, if 

 any one of them, as frequently happens, exhibits a marked 

 advantage over its competitors, I select the paper which bears 

 the corresponding number to be placed in the camera obscura. 



(2.) Method qfjixing the images. — After having tried am- 

 monia, and several other reagents, with very imperfect success, 

 the first thing which gave me a successful result was the iodide 

 of potassium, much diluted with water. If a photogenic picture 

 is washed over with this liquid, an iodide of silver is formed 

 which is absolutely unalterable by sunshines. This process re- 

 quires precaution ; for if the solution is too strong, it attacks 

 the dark parts of the picture. It is requisite, therefore, to find 

 by trial the proper proportions. The fixation of the pictures 

 in this way, with proper management, is very beautiful and 

 lasting. The specimen of lace which I exhibited to the So- 

 ciety, and which was made five years ago, was preserved in 

 this manner. 



But my usual method of fixing is different from this, and 

 somewhat simpler, or at least requiring less nicety. It con- 

 sists in immersing the picture in a strong solution of common 

 salt, and then wiping off the superfluous moisture, and drying 

 it. It is sufficiently singular that the same substance which 

 is so useful in giving sensibility to the paper, should also be 

 capable, under other circumstances, of destroying it ; but such 

 is, nevertheless, the fact. 



