12

NEGATIVE l’HOTOGUAPHS.

I select, in the first place, paper of a good firm quality and smoothsurface. I do not know that any answers better than superfine writingpaper. I dip it into a weak solution of common salt, and wipe it dry, bywhich the salt is unifonnly distributed throughout its substance. Ithen spread a solution of nitrate of silver on one surface only, and dryit at the fire. The solution should not be saturated, but six or eighttimes diluted with water. When dry, the paper is fit for use.

“ I have found by experiment that there is a certain proportionbetween the quantity of salt and that of the solution of silver whichanswers best, and gives the maximum effect. If the strength of tliosalt is augmented beyond this point, the effect diminishes, and, in cer-tain cases, becomes exceedingly small.

“ This paper, if properly made, is very useful for all ordinary photo-genic purposes. For example, nothing can bo more perfect than theimages it gives of leaves and flowers, especially with a summer sun,—thelight, passing through the leaves, delineates every ramification of theirnerves.

“ Now, suppose we take *a sheet thus prepared, and wash it with asaturated solution of salt, and then dry it. Wo shall find (especially ifthe paper has been kept some weeks before the trial is made) that itssensibility is greatly diminished, and, in some cases, seems quite extinct.But if it is 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, anddrying it between times, I have succeded in increasing its sensibility tothe degree that is requisite for receiving the images of the cameraobscura.

“ In conducting this operation, it will bo found that tlio results aresometimes more and sometimes less satisfactory, in consequence of smalland accidental variations in tlio proportions employed. It happenssometimes that tlio chloride of silver is disposed to darken of itselfwithout any exposure to light; this shows that the attempt to give itsensibility has been carried too far. The object is to approach to thiscondition as near as possible without reaching it, so that the substancomay be in a state ready to yield to the slightest extraneous force, suchas the foeblo impact of the violet rays when much attenuated. Havingtherefore prepared a number of sheets of paper, with chemical propor-tions slightly different from one another, let a piece be cut from each,and having been duly marked or numbered, let them be placed, side byside, in a very weak diffused light for a quarter of an hour. Then, ifany one of them, as frequently happens, exhibits a marked advantageover its competitors, I select the paper which bears the correspondingnumber to bo placed in the camera obscura.”*

In this extract from Mr. Talbot’s communication, wo have enumer-ated, in brief, most of the peculiarities of the photographic processes:

The London and Edinburgh Philosophical Magazine, March, 1839, page 209, vol. 14.