1^5 



perfine writing paper. He dips it into a weak solution of common 

 salt, and wipes it dry, by which the salt is uniformly distributed 

 throughout its substance. He then spreads a solution of nitrate of 

 silver on one surface only, and dries it at the fire. The solution 

 should not be saturated, but six or eight times diluted with water. 

 When dry, the paper is fit for use. He has found by experiment 

 that there is a certain proportion between the quantity of salt and 

 that of the solution of silver which answers best, and gives the 

 maximum effect. If the strength of the salt is augmented beyond 

 this point, the effect diminishes, and in certain cases becomes ex- 

 ceedingly small. This paper, if properly made, is very useful for all 

 ordinary photogenic purposes. For example, nothing can be more 

 perfect than the images it gives of leaves and flowers, especially with 

 a summer sun. The light passing through the leaves delineates 

 every ramification of their nerves. If a sheet of paper thus prepared, 

 be taken and washed with a saturated solution of salt, and then 

 dried, it will be found (especially if the paper has been kept some 

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

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

 with a liberal quantity of the solution of silver, it becomes 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, Mr. Talbot has succeeded in increasing its sensibility 

 to the degree that is requisite for receiving the images of the camera 

 obscura. In conducting this operation it will be found that the re- 

 sults are sometimes more and sometimes less satisfactory in conse- 

 quence of small and accidental variations in the proportions employ- 

 ed. It happens sometimes that the chloride of silver is disposed to 

 darken of itself without any exposure to the light : this shows that 

 the attempt to give it sensibility has been carried too far. The object 

 is, to approach to this condition as near as possible, v^AXhout 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 

 t)f sheets of paper slightly different from one another in the composi- 

 tion, 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 compe- 

 titors, Mr. Talbot selects the paper which bears the corresponding 

 number, to be placed in the camera obscura. 



With regard to the second object, that of fixing the images, Mr. 

 Talbot observed that after having tried ammonia and several other re- 

 agents, with very imperfect success, the first which gave him a suc- 

 cessful 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 sunshine. This 

 process requires precaution ; for, if the solution is too strong, it at- 

 tacks 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 



