segunda-feira, 29 de março de 2010

Philosophical Magazine and Journal of Science



Philosophical Magazine


Journal of Science

(Third series)


Pag.88, 89, 90, 91, 92









Richard Philips,

F.R.S.L.&E. F.G.S. &C.













July 1841


XVII. Intelligence and Miscellaneous Articles.




Dear sir,—IT is now two years since I first published a brief account of Photogenic Drawing. During this interval I have taken much pains, and made many experiments, with the hope of rendering the art more perfect and useful. In this way I have obtained a good many improvements, with the mention of which I shall not detain you at present.

I shall confine myself in this letter to a single subject, viz. the discovery which I made last September of a chemical process by which paper may be made far more sensitive to light than by any means hitherto known. It is not easy to estimate exactly how far this increase of sensibility extends ; but certainly a much better picture can now be obtained in a minute than by the former process in an hour.

This increased rapidity is accompanied with an increased sharpness and distinctness in the outlines of the objects—an effect which is very advantageous and pleasing, and at the same time rather difficult to account for.

The shortest time in which I have yet succeeded in impressing an image in the camera obscura has been eight seconds; but I do not mean to assign this as the precise limit, for it can only be ascertained by more careful and multiplied experiments.

The production of the image is accompanied with some very extraordinary circumstances, to which I will advert in a subsequent letter. These phænomena are extremely curious; and I have not found in chemical writers any mention of anything similar.

The image, when obtained, must of course be fixed, otherwise the process would remain imperfect. It might be supposed, à priori, that this fixation would be very difficult, the paper being so sensitive. But it fortunately happens that, in this instance, what seems a reasonable inference is not borne out by fact, that new photographs are more easily and perfectly fixed than was the case with the former ones. When fixed, a great many copies may be made from them ; and thus the original view can be multiplied with facility.

I think that the art has now reached a point which is likely to make it extensively useful. How many travellers are almost ignorant of drawing, and either attempt nothing, or bring home rude unintelligible sketches ! They may now fill their portfolios with accurate views, without much expenditure of time or trouble; and even the accomplished artist will call in sometimes this auxiliary aid, when pressed for time in sketching a building or a landscape, or when wearied with the multiplicity of its minute details.

One of the most important applications of the new process, and most likely to prove generally interesting, is, undoubtedly, the taking of portraits. I made trial of it last October, and found that the experiment readily succeeded. Half a minute appeared to be sufficient in sunshine, and four or five minutes when a person was seated in the shade, but in the open air. After a few portraits had been made, enough to show that it could be done without difficulty, the experiments were adjourned to a more favourable season.

Several photographic processes being now known, which arc materially different from each other, I consider it to be absolutely necessary to distinguish them by different names, in the same way that we distinguish different styles of painting or engraving. Photographs executed on a silver plate have received, and will no doubt retain, the name of Daguerréotype. The new kind of photographs, which are the subject of this letter, I propose to distinguish by the name of Calotype ; a term which, I hope, when they become known, will not be found to have been misapplied.

I remember it was said by many persons, at the time when photogenic drawing was first spoken of, that it was likely to prove injurious to art, as substituting mere mechanical labour in lieu of talent and experience. Now, so far from this being the case, I find that in this, as in most other things, there is ample room for the exercise of skill and judgement. It would hardly be believed how different an effect is produced by a longer or shorter exposure to the light, and, also, by mere variations in the fixing process, by means of which almost any tint, cold or warm, may be thrown over the picture, and the effect of bright or gloomy weather may be imitated at pleasure. All this falls within the artist's province to combine and to regulate; and if, in the course of these manipulations, he, nolens volens, becomes a chemist and an optician, I feel confident that such an alliance of science with art will prove conducive to the improvement of both.

31, Sackville Street, Feb. 5,1841.

I remain, yours, &c.

H. F. Talbot.

Dear Sir,—I will now proceed to give you some further details, for which I had not room in my last letter, respecting the phænomena which occur during the very singular photographic process to which I have given the name of Calotype. And I may as well begin by relating to you the way in which I discovered the process itself. One day, last September, I had been trying pieces of sensitive paper, prepared in different ways, in the camera obscura, allowing them to remain there only a very short time, with the view of finding out which was the most sensitive. One of these papers was taken out and examined by candlelight. There was little or nothing to be seen upon it, and I left it lying on a table in a dark room. Returning some time after, I took up the paper, and was very much surprised to see upon it a distinct picture. I was certain there was nothing of the kind when I had looked at it before; and, therefore (magic apart), the only conclusion that could be drawn was, that the picture had unexpectedly developed itself by a spontaneous action.

Fortunately, I recollected the particular way in which this sheet of paper had been prepared, and was, therefore, enabled immediately to repeat the experiment. The paper, as before, when taken out of the camera, presented hardly anything visible ; but this time, instead of leaving it, I continued to observe it by candlelight, and had soon the satisfaction of seeing a picture begin to appear, and all the details of it come out one after the other.

In this experiment, the paper was used in a moist state; but since it is much more convenient to use dry paper if possible, I tried it shortly afterwards in a dry state, and the result was still more extraordinary. The dry paper appeared to be much less sensitive than the moist; for when taken out of the camera after a short time, as a minute or two, the sheet of paper was absolutely blank.

But, nevertheless, I found that the picture existed there, although invisible; and by a chemical process analogous to the foregoing, it was made to appear in all its perfection.

After several further experiments, which were requisite in order to come to a right understanding of this unexampled natural process, I found it expedient to abandon the former method of taking views with the camera, in favour of the new one, so far excelling it in rapidity and power. The result of my experience hitherto with this calotype paper is, that if properly prepared, it will keep three or four months, ready for use at any moment, and moreover it is used in a dry state, which is a great convenience.

The time of exposure to light in the camera may be varied, according to circumstances, from a quarter of a minute upwards; and the paper, when taken out of the instrument, appears quite blank, as I said before, but it is impressed with an invisible image. It may be kept in this invisible state for a month or so, if desired, and brought out, or rendered visible, when wished for. But, generally, this is done shortly after, or at least on the same day, for fear of accidents (such as a casual gleam of daylight, which would at once annihilate the whole performance). Whenever it is desired to render the picture visible, this is done in a very short time, as from a minute to five or ten minutes, the strongest impressions coming out the easiest and quickest. Very faint impressions (as those obtained when the paper has been only a few seconds in the camera, or the objects have not been luminous enough) take a longer time in coming out; but they should not be despaired of too soon, as many of them exhibit difficulty at first, as if reluctant to appear, but nevertheless end by coming out very well. The operator of course remains in a darkened room, lit by candles only.

I know few things in the range of science more surprising than the gradual appearance of the picture on the blank sheet, especially the first time the experiment is witnessed. The operator ought to watch the progress of the picture, until, in its strength of colour, sharpness of outline, and general distinctness, it has reached in his judgement the most perfect state. At that moment he stops further progress by washing it over with a fixing liquid. This is washed off with water, the picture is then dried, and the process is terminated.

The picture is found to be very strongly fixed, and from it numerous copies may be taken on common photogenic drawing paper, by the method of superposition in sunshine. The original picture does not readily become altered, or wear out by this exposure to the sun ; but in case it does so, as happens sometimes, I find that it may be in general readily revived. This revival, which is a most curious particularity of the calotype process, not only restores the picture to its pristine strength, but frequently causes fresh details and minutiæ to appear in the picture, which had not appeared before, at the time when it was first brought out, or rendered visible (owing to that process having been checked too soon). These details, therefore, had been lying in an invisible state on the paper all this time, not destroyed (which is the most extraordinary thing) by so much exposure to sunshine. They were protected by the fixing liquid. But no one could have supposed beforehand, or without ocular demonstration, that it could have exerted so complete a protecting power. This is an invaluable property of the calotype—the power of reviving the pictures—not only because it allows so many copies to be made, but because it enables the artist to correct the error of his judgement, in case he has made too faint a picture at first, by stopping it too soon while it was coming out.

Some further details on this subject, and an account of the chemical processes employed, I reserve for a paper which I intend to lay before the Royal Society.

I am, &c. &c.

Lacock Abbey, Feb. 19, 1841,

H. F. Talbot.

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