domingo, 29 de agosto de 2010



21 de Fevereiro





Vol. IV

1837 to 1843



Pag. 124, 125, 126



Nº. 37

A paper was read. Entitled, “ An Account of the Processes employed in Photogenic Drawing,” in a letter to S. Hunter Christie, Esq . Sec. R. S. By H. Fox Talbot, Esq., F.R.S.

The subject, Mr. Talbot observes, naturally divides itself into two heads, - the preparation of the paper, and the means of fixing the design.

In order to make what may be called ordinary Photogenic paper, the author selects, in the first place, paper of a good firm quality and smooth surface; and thinks that none answers better than superfine writing paver. 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 efect. If the strength of the salt is augmented beyond this point, the effect diminishes and in certain cases becomes exceedingly 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 diminished, and in some cases seems quite extinct. But if it be again washed with a liberal quantity of the solation 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 results are sometirnes more and sometimes less satisfactory in consequence of small and accidental variations in the proportion employed. It happens sometimes that the chloride of silver is disposed to darken of itself without any esposure 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, without reaching it, so that the substance may be in a state ready to yield to the slightest extrancous force, such as the feeble impact of the violet rays when much attenuated. Having therefore prepared a number of sheets of paper slightly different from one another in the composh cut from cach, 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, 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 that after having tried ammonia and several other reagents, with very imperfect success, the first which gave him a succesful 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 attacks the dark parts of the picture. It is requisite therefore to find by trail proper proportions. The fixation of the pictures in this way, with proper management, is very beautiful and lasting. The specimen of lace which Mr. Talbot exhibited to the Society, and which was made fire years ago, was preserved in this manner. But his usual method of fixing is different from this, and somewhat simpler, or at least, requiring less nicety. It consists in immersing the picture in a strong solution of common salt, and then wiping off the superfltious 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. Now, if the picture which has been thus washed and dried, is placed in the sun, the white parts colour themselves of a pale lilac tint; after which they become insensible.

Numerous experiments have shown the author, that the depth of this lilac tint varies according to the quantity of salt used relatively to the quantity of silver. But by properly adjusting these, the images may, if desired, be retained of an absolute whiteness. He mentions also, that those preserved by iodine are always of a very pale primrose yellow, which has the extraordinary and very remarkable property of turning to a full gaudy yellow whenever it is exposed to the heat of a fire, and recovering its, former colour again when it is cold.

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