Page:Proceedings of the Royal Society of London Vol 4.djvu/149

Rh 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