23 de Fevereiro
THE LITERARY GAZETTE AND JOURNAL OF BELLES LETTRES, ARTS, SCIENCES
London, Saturday, february 23, 1839
pag. 123, 124
after routine business, amongst which we noticed the election of Lieut. Col. Reid, the author of the very interesting “Theory of Storms”, the following letter from Mr. Fox Talbot o the secretary, was read by Mr. Christie:-
Dear Sir, - in compliance with the request of several scientific friends, who have been much interested with the accounte of the art of Photogenic Drawing, which I had the honour of presenting to the Royal Society on the 31 st of last month, I will endavour to esplain, as briefly as I can, but at the same time without omitting any thing essencial, the methods which I have hitherto empoyed for the production of these pictures.
If this explanation, on my parte, should have the effect of drawing new inquirers into the field, and if any new discoveries of importance should be the result, as I anticipate, and especially if any means should be discovered by which the sensitiveness of the paper can be materially increased, I shall be the first to rejoice at the success; and, in the meanwhile, I shall endeavour, as far as I may be able to prosecut the inquiry myself.
The subject naturally divides itself into two heads; viz, the preparation of the paper, and the means of fxing the design.
(1.) Preparation of the paper – In order to make what may be called ordinary photogenic paper, I select, in the first place, paper of a good firm quality and smooth surface. I do not know that any answers better than superfine writing paper. I dip it into a weak solution of common salt, and wipe it dry, by which the salt is uniformly distributed throughout its substance. I then spread a solution of nitrate of silver on one surface only, and dry 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.
I have 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 exceedingly small.
This paper, if properly made, is very useful for all ordinary photogenic pruposes. 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.
Now, suppose we take a sheet of paper thus prepared, and wash it with a satured solution of salt, and then dry it. We shall find (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 is again washed with a liberal quantity of the solution of silver, it becomes again sensible to light, and evenit; so that the more so than it was at first. In this way, by alternately washingthe paper with salt and silver, and drying it between times, I have succeeded in increasing its sensibility to the degree that is requisit for receiveing the images of the camera obscura.
In conducting this oeration it will be found that the results are sometimes more anr sometimes less satisfactory in consequence 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 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 extraneous force, such as the feeble impact of the violet rays whwn 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, axhibits a marked advantage over its competitors, I select the paper which bears the corresponding number to be placed in the camera obscura.
(2.) Method of fixing the images. – after having tried ammonia, and several other reagents, with very imperfect success, the first thing which gave me a successful resultwas the iodide of potassium, much diluted with water. If a photogenic picture is washed over with this liquid, an iodid 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 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 Society, 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 consists in immersing the pictures 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 others circumstances, of destroying il; 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 to me 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 whitneness. I find I have omitted to mention that those preserved by iodine are always of a very pale primorose 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. – I am, & c.
H. Fox Talbot
44 Queen Ann Street, Feb. 20th, 1839.
We are much pleased with the frank and ingenuous manner in which our countryman has come forward to give publicity to this process, and state the results of this experiments.this is the way to promote the general benefit, and lead others into the méthode of pursuing similar inquires, by wich the discovery may be improved and perfect. In this class we rejoice to learn that Sir John Herschel has devoted his attention to the subject, and has already, we understand, made curious progress, inasmuch as he has obtained the pictures from the light of Daniell’s great galvanic battery. Sir David Brewster too, we are informed, has taken up the investigation; and when such men set to work, we may look for much to follow.
Before laying down our pen, we should mention that, at the Royal Society, Mr. Talbot shewed us the perfect picture of a riband, some three inches broad, and of a ribbed and watered pattern, taken in this manner, but not by the sun, the only active agent being the common day light !! and in a London atmosphere of the month of February too. After this, who can doubt the extreme sensibility of the prepared paper? – Ed. L.G.