31 de Janeiro
É lido, na Royal Society, o texto da autoria de Henry Fox Talbot
Some Account of the Art of Photogenic Drawing
Or, the process by which Natural Objects may be made to delineate themselves, without the aid of the Artist’s Pencil.
In the spring of 1834 I began to put in practice a method which I had devised some time previously, for employing to purposes of utility the very curious property which has been long known to chemists to be possessed by the nitrate of silver: namely its discoloration when exposed to the violet rays of light. This property appeared to me to be perhaps capable of useful application in the following manner.
I proposed to spread on a sheet of paper a sufficient quantity of the nitrate of silver; & then to set the paper in the sunshine, having first placed before it some object casting a well defined shadow.
The light, acting on the rest of the paper would naturally blacken it, while the parts in shadow would retain their whiteness. Thus I expected that a kind of image or picture would be produced, resembling to a certain degree the object from which it was derived.
I expected however also, that it would be necessary to preserve such images in a portfolio, & to view them only by candlelights: because, if by daylight, the same natural process which formed the images, would destroy them, by blackening the rest of the paper.
Such was my leading idea, before it was enlarged & corrected by experience.
It was not until some time after, & when I was in possession of several novel & curious results, that I thought of enquiring whether this process had been ever proposed or attempted before?
I found that in fact it had: but apparently not followed up to any extent, or with much perseverance, the few notices that I have been able to meet with, one vague and unsatisfactory; merely stating that such a method exists of obtaining the outline of an object, but going into no details respecting the best & most advantageous manner of proceeding.
The only definite account of the matter which I have been able to meet with, is contained in the first volume of the Journal of the Royal Institution page 170. from which it appears that the idea was originally started by Mr. Wedgwood, and a numerous series of experiments made both by him & Sir Humphry Davy, which however ended in failure.
I will take the liberty of quoting a few passages from this memoir.
“The copy of a painting, immediately after being taken, must be kept in an obscure place. It may indeed be examined in the shade, but in this case the exposure should be only for a few minutes.
No attempts that have been made to prevent the uncoloured parts from being acted upon by light, have as yet been successful. They have been covered with a thin coating of fine varnish, but this has not destroyed their susceptibility of becoming coloured.
When the solar rays are passed through a print & thrown upon prepared paper, the unshaded parts are slowly copied: but the lights transmitted by the shaded parts, are seldom so definite as to from a distinct resemblance of them, by producing different intensities of colour.
The images found by means of a camera obscura have been found to be too faint to produce in any moderate time, an effect upon the nitrate of silver. To copy these images was the first object of Mr. Wedgwood, but all his numerous experiments proved unsuccessful.”
These are the observations of Sir Humphry Davy. I have been informed by a scientific friend that this unfavourable result of Mr. Wedgwood’s & Sir Humphry Davy’s experiments, was the chief cause which discouraged him from following up with perseverance, the idea, which he had also entertained of fixing the beautiful images of the camera obscura.
And no doubt, when so distinguished an experimenter as Sir H. Davy announced “that all experiments had proved unsuccessful” such a statement was calculated materially to discourage further inquiry.
The circumstance also, announced by Davy, that the paper on which these images were depicted, was liable to become entirely dark; & that nothing hitherto tried, would prevent it; would perhaps have induced one to consider the attempt as hopeless - if I had not (fortunately) before I read it, already discovered a method of overcoming this difficulty, & of fixing the image in such a manner, that it is no more liable to injury or destruction.
In the course of my experiments directed to that end, I have been astonished at the variety of effects which I have found produced by a very limited number of different processes when combined in various ways; and also at the length of time which sometimes elapses before the fade effect of these manifests itself with certainty.
For, I have found, that images formed in this manner, which have appeared in good preservation at the end of 12 months from the time of their formation, have nevertheless somewhat altered during the second year. This circumstance added to the fact that the first attempts which I made, became indistinct in process of time (the paper growing wholly dark) induced me to watch the process of the change during some considerable time: as I thought that perhaps all these images would ultimately be found to fade away.
I found however to my satisfaction, that this was not the case: and having now kept a number of these drawings during nearly five years, without their suffering any deterioration I think myself authorised to draw conclusions from my experiments with more certainty.
Effect & appearance of these images
The images obtained in this manner are themselves white, but the ground upon which they display themselves is variously & pleasingly coloured.
Such is the variety of which the process is capable, that by merely varying the proportions & some trifling details of manipulation, any of the following colours are readily obtainable
Brown, of various shades
Green alone, is absent from the list, with the exception of a dark shade of it, approaching to black.
The blue-coloured variety has a very pleasing effect, somewhat like that produced by the Wedgewood-ware which has white figures on a blue ground. This variety also retains its colours perfectly if preserved in a portfolio, & not being subject to any spontaneous change requires no preserving process.
These different shades of colour are of course so many different chemical compounds which chemists have not hitherto distinctly noticed.
First application of this process.
The first kind of objects which I attempted to copy by this process were flowers & leaves, either fresh or selected from my herbarium.
These it renders with the utmost truth & fidelity - exhibiting even the venation of the leaves, the minute hairs that clothe the plant, &c. &c.
It is so natural to associate the idea of labour with great complexity and elaborate detail of execution; that one is more struck, at seeing the thousand florets of an Agrostis depicted with all its capillary branchlets (and so accurately, that none of all this multitude shall want its little bivalve calyx requiring to be examined through a lens) - than one is by the picture of the large and simple leaf of an oak or chesnut. But in truth the difficulty is in both cases the same.
The one of these takes no more time to execute than the other: for, the object which would take the most skilful artist days or weeks of labour to trace or to copy, is effected by the boundless powers of natural chemistry in the space of a few seconds.
To give an idea of the degree of accuracy with which some object could be initiated by this process, I need only mention one instance.
Upon one occasion, having made an image of a piece of lace of an elaborate pattern, I showed it to some persons, at the distance of a few feet, with the enquiry: whether it was a good representation?
When the reply was: “that they were not to be so easily deceived; for that was evidently no picture, but the piece of lace itself."
At the very commencement of my experiments upon this subject, when I saw how beautiful were the images which were thus produced by the action of light, I regretted the more that they were destined to have such a brief existence, & I resolved to attempt to find out if possible some method of preventing this, or retarding it as much as possible.
The following Considerations led me to conceive the possibility of discovering a preservative process.
The nitrate of silver which has become black by the action of light, is no longer the same chemical substance that it was before. Consequently, if a picture, produced by solar light, is subjected afterwards to any chemical process, the white & dark parts of it will be differently acted upon: & there is no evidence, that after this action has taken place, these white & dark parts will any longer be subject to a spontaneous change. Or, if they are so, still it does not follow that that change will now tend to assimilate them to each other.
In case of their remaining dissimilar the picture will remain visible, & therefore our object will be accomplished.
If it should be asserted that exposure to sunshine would necessarily reduce the whole to one uniform tint, & destroy the picture, the onus probandi evidently lies on those who make the assertion. If we designate by the letter A the exposure to the solar light; and by B some indeterminate Chemical process, my argument was this: - Since it cannot be shown à priori that the final result of the series of processes A. B. A. will be the same with that denoted by B. A. it will therefore be worth while to put the matter to the test of experiments: - viz. by varying the process B until the right one be discovered: or until so many trials have been made as to preclude all reasonable hope of its existence. My first trials were unsuccessful, as indeed I expected: but after some time I discovered amethod which answers perfectly, and shortly afterwards, another.
On one of these more especially I have made numerous experiments: the other I have comparatively little used, because it appears to require more nicety in the management. It is however equal, if not superior, to the first in brilliancy of effect.
This chemical change, which I call the preserving process, is far more effectual than could have been anticipated. The paper which had previously been so sensitive to light, becomes completely insensitive to it, insomuch that I am able to show the Society specimens which have been exposed for an hour to the full summer sun, & from which exposure the image has suffered nothing, but retains its perfect whiteness.
On the Art of Fixing a Shadow
The phenomenon which I have now briefly mentioned appears to me to partake of the character of the marvellous, almost as much as any fact which physical investigation has yet brought to our knowledge.
The most transitory of things - a shadow - the proverbial emblem of all that is fleeting and momentary – may be fettered by the spells of our “natural magic”, & may be fixed for ever in the position which it seemed only destined for a single instant to occupy.
This remarkable phenomenon, of whatever value it may turn out in its application to the arts, will at least be accepted as a new proof of the value of the inductive methods of modern science, which by noticing the occurrence of unusual circumstances (which accident perhaps first manifests in some small degree) & by following them up with experiments, & varying the conditions of these, until the true law of nature which they express, is apprehended; conducts us at length to consequences altogether unexpected, remote from usual experience, & contrary to almost universal belief. Such is the fact, that we may receive on paper the fleeting shadow; arrest it there; & in the space of a single minute fix it there so firmly as to be no more capable of change, even if thrown back into the sunbeam from which it derived its origin.
Before going further I may however add, that it is not always necessary to use a preserving process. This I did not discover until after I had acquired considerable practice in this art; having supposed at first that all these pictures would ultimately become indistinct if not preserved in some way from the change.
But experience has shown to me that there are at least two or three different ways in which the process may be conducted so that the images shall possess a character of durability, provided they are kept from the action of direct sunshine. These ways have presented themselves to notice rather accidentally than otherwise: in some instances without any particular memoranda having been made at the time; so that I am not yet prepared to state accurately, on which particular thing this sort of semi-durability depends, or what cause is best to be followed in order to obtain it. But as I have found that certain of the images which have been subjected to no preserving process, remain quite white and perfect after the lapse of a year or two, & indeed show no symptom whatever of changing: which others differently prepared (& left unpreserved) have grown quite dark in one tenth of that time – I think this singularity requires to be pointed out. Whether it will be of much value I do not know: perhaps it will be thought better to incur at first the small additional trouble of employing the preserving process, especially as the drawings thus prepared will stand the sunshine; while the unpreserved ones, however well they last in a portfolio or in common daylight, should not be risked in a very strong light, as they would be liable to change thereby even years after their original formation. This very quality however admits of useful application. For, this semidurable paper, which retains its whiteness for years in the shade, & yet suffers a change whenever exposed to the solar light, is evidently well suited to the use of a naturalist travelling in a distant country, who may wish to keep some memorial of the plants he finds, without having the trouble of drying them & carrying them about with him. He would only have to take a sheet of this paper, throw the image upon it & replace it in his portfolio.
The defect of this particular paper is, that in general the ground is not even; but this is of no consequence where utility alone & not beauty of effect, is consulted.
Another purpose for which I think my method will be found very convenient is the making of outline portraits, or silhouettes. These are now often traced by the hand, from shadows projected by a candle. But the hand is liable to err from the true outline, & a very small deviation causes a notable diminution in the resemblance. I believe this manual process cannot be compared with the truth and fidelity with which the portrait is given by means of solar light.
Paintings on glass
The shadow pictures which are found by exposing paintings on glass to solar light, are very pleasing. The glass itself, around the painting, should be blackened: such, for instance as are often employed for the magic lantern. The paintings on the glass should have no bright yellows or reds, for these stop the violet rays of light which are the only effective ones. The pictures thus formed resemble the productions of the artist’s pencil more, perhaps, than any of the others. Persons to whom I have shown them have generally mistaken them for such, at the same time observing, that the style was new to them, & must be one rather difficult to acquire.
It is in these pictures only, that as yet, I have observed indications of Colour. I have not had time to pursue this branch of the enquiry further. It would be a great thing if by any means we could accomplish the delineation of objects in their natural colours. I am not very sanguine respecting the possibility of this: yet, as I have just now remarked, it appears possible to obtain at least some indication of variety of tint.
Application to the Microscope
I now come to a branch of the subject which appears to be very important and likely to prove extensively useful - the application of my method of delineating objects, to the solar microscope.
The objects which the microscope unfolds to our view, curious & wonderful as they are, are often singularly complicated. The eye indeed may comprehend the whole which is presented to it, in the field of view: but the powers of the pencil fail to express these minutiae of nature in their innumerable details. What artist could have skill or patience enough to copy them? Or, granting that he could do so, must it not be at the expense of much most valuable time, which might be more usefully employed?
Contemplating the beautiful picture which the solar microscope produces, the thought struck me whether it might not be possible to cause that image to impress itself upon the paper, & thus to let Nature substitute her own imitable pencil, for the imperfect tedious & almost hopeless attempt of copying a subject so intricate?
My first attempt had no success. Although I chose a light day, & formed a good image of my object, upon prepared paper; on returning at the expiration of an hour I found that no effect had taken place – I was thereupon half inclined to abandon this experiment, when it occurred to me, that there was no reason to suppose that the common nitrate of silver was the most sensitive substance that exists, to the action of the Chemical Rays: and though such should eventually prove to be the fact, at any rate it was not to be assumed without proof.
I therefore began a course of experiments in order to ascertain the influence of various modes of preparation, & I found these to be signally different in their results. I considered this matter chiefly in a practical point of view; for as to the theory, I confess that I cannot as yet understand the reason why the paper prepared in one way should be so much more sensitive, than in another.
The result of these experiments was the discovery of a mode of preparation vastly superior in sensibility to what I had originally employed: & by means of this, all those effects which I had before only anticipated as theoretically possible were found to be capable of realization.
When a sheet of this, which I shall call “Sensitive Paper” is placed in a dark chamber & the magnified image of some object thrown on it by the solar microscope; after the lapse of perhaps a quarter of an hour, the picture is found to be completed. I have not as yet used high magnifying power, on account of the consequent enfeeblement of the light. Of course, with a more sensitive paper, greater magnifying power will become desirable.
On examining one of these pictures, which I made about 3 years & a half ago, I find, by actual measurement of the picture and the object, that the latter is magnified 17 times in linear diameter, & in surface consequently 289 times. I have others which I believe are considerably more magnified; but I have lost the corresponding objects, so that I cannot here state the exact numbers.
Not only does this process save our time & trouble, but there are many objects, especially microscopic crystallisations, which alter so greatly in the cause of 3 or 4 days (and it could hardly take any artist less to delineate them in all their details) that they could never be drawn in the usual way.
I will now describe the degree of sensitiveness which this paper possesses - premising that I am far from supposing that I have reached the limit of which this quality is capable. On the contrary, considering the few experiments which I have made, (few, that is, in comparison with the number which it would be easy to imagine & propose) I think it most likely, that other methods may be found, by which substances may be prepared perhaps as much transcending in sensitiveness the one which I have employed, as that does the ordinary state of the nitrate of silver.
But, to confine myself to what I have actually accomplished, in the preparation of a very sensitive paper.
When a sheet of this paper is brought towards a window, not one through which the sun shines, but looking in the opposite direction, it immediately begins to discolour.
For this reason, if the paper is prepared by sunlight, it must by no means be left uncovered, but as soon as finished be shut up in a drawer or cupboard & there left to dry: or else, dried at night by the warmth of a fire.
Before using this paper for the delineation of any object, I generally approach it for a little time towards the light, thus intentionally giving it a slight shade of colour, for the purpose of seeing that the ground is even. If it appears so, when thus tried to a small extent, it will generally be found to prove so in the final result. But if there are some places or spots in it which do not acquire the same tint as the rest, such a sheet of paper should be rejected: for there is a risk that when employed – instead of presenting a ground uniformly dark, which is essential to the beauty of the drawing, it will have large white spots, - places altogether insensible to the effect of light.
This singular circumstance I shall revert to elsewhere: it is sufficient to mention it here:
The paper then, which is thus readily sensitive to the light of a common window, is of course much more so, to the direct sunshine. Indeed, such is the velocity of the effect then produced, that the picture may be said to be ended almost as soon as it is begun.
To give some more definite idea of the rapidity of the process, I will state, that after various trials the nearest evaluation which I could make of the time necessary for obtaining the picture of an object so as to have pretty distinct outlines, when I employed the full sunshine, was half a second.
Architecture, Landscape, & external Nature
But perhaps the most curious application of this art is the one I am now about to relate. At least, it is that which has appeared the most surprising to those who have examined my collection of pictures formed by solar light.
Everyone is acquainted with the beautiful effects which are produced by a Camera Obscura & has admired the vivid picture of external nature which it displays. It had often occurred to me, that if it were possible to retain upon the paper the lovely scene which thus illuminates it for a moment; or if we could but fix the outlines of it, the lights & shadows: divested of all colour; such a result could not fail to be most interesting. And however much I might be disposed at first to treat this notion as a scientific dream, yet when I had succeeded in fixing the images of the Solar Microscope by means of a peculiarly sensitive paper, there appeared no longer any doubt that an analogous process would succeed in copying the objects of external nature although indeed they are much less illuminated.
Not having with me in the country a camera obscura of any considerable size, I constructed one out of a large box, the image being thrown upon one end of it by a good object glass fixed in the opposite end.
This apparatus being armed with a sensitive paper was taken out in a summer afternoon & placed about 100 yards from a building favourably illuminated by the sun. An hour or two afterwards I opened the box, & I found depicted upon the paper a very distinct representation of the building, with the exception of those parts of it which were in the shade.
A little experience in this branch of the art showed me, that with smaller camera obscuræ the effect would be produced in a smaller time.
Accordingly I had several small boxes made, in which I fixed lenses of shorter focus, and with these I obtained very perfect but extremely small pictures: such as without great stretch of imagination might be supposed to be the work of some Lilliputian artist. They require indeed examination with a lens, to discover all their minutiæ.
In the summer of 1835 I made in this way a great number of representations of my house in the country; which is well suited to the purpose, from its ancient & remarkable architecture. And this building I believe to be the first, that was ever yet known to have drawn its own picture.
The method of proceeding was this: - Having first adjusted the paper to the proper focus, in each of these little cameræ, I then took a number of them with me out of doors, & placed them in different situations around the building. After the lapse of half an hour I gathered them all up, & brought them within doors to open them. When opened, there was found in each a miniature picture of the objects before which it had been placed.
To the traveller in distant lands, who is ignorant as too many unfortunately are, of the art of drawing, this little invention may prove of real service. And even to the artist himself, however skilful he may be. For although this natural process does not produce an effect much resembling the productions of his pencil; & therefore cannot be considered as capable of replacing them: yet it is to be recollected that he may often be so situated, as to be able to devote only a single hour to the delineation of some very interesting locality. Now, since nothing prevents him from simultaneously disposing, in different positions, any number of these little cameræ, it is evident that their collective results, when examined afterwards, may furnish him with a large body of interesting memorials, & with numerous details which he had not had himself time either to note down or to delineate.
Delineations of sculpture.
Another use which I propose to make of my invention is for the copying of statues & basreliefs. I place these in strong sunshine, & put before them at a proper distance & in the requisite position a small camera obscura containing the prepared paper. In this way I have obtained images of various statues &c. I have not pursued this branch of the subject to any extent: but I expect interesting results from it & that it may be usefully employed under many circumstances.
Copying of Engravings
The invention may be employed with great facility for obtaining copies of drawings, or engravings, or facsimiles of M.S.S.
For this purpose the engraving is pressed upon the prepared paper, with its engraved side in contact with the latter. The pressure must be as uniform as possible, that the contact may be perfect: for, the least interval sensibly injures the result, by producing a kind of clouding in lieu of the sharp strokes of the original.
When placed in the sun, the solar light gradually traverses the paper, except in those placed where it is prevented from doing so by the opaque lines of the engraving. It therefore of course makes an exact images or print of the design. This is one of the experiments which Davy and Wedgwood state that they tried, but failed, from want of sufficient sensibility in their paper.
The length of time requisite for effecting the copy depends on the thickness of the paper onwhich the engraving has been printed.
At first I thought that it would not be possible to succeed with thick papers: but I found on trial that the success of the method was by no means so limited. It is enough for the purpose, if thepaper allows any of the solar light to pass.
Then the paper is thick, I allow half an hour for the formation of a good copy. In this way I have copied very minute complicated & delicate engravings, crowded with figures of small size, which were rendered with great distinctness.
The effect of the copy, though of course unlike the original, (substituting as it does lights for shadows and vice versa) yet is often very pleasing, & would I think suggest to artists useful ideas respecting light & shade.
It may be supposed that the engraving would be soiled or injured by being thus pressed against the prepared paper. There is not much danger of this, provided both are perfectly dry. It may be well to mention however, that in case any stain should be perceived on the engraving it may be readily removed by a chemical application which does no injury whatever to the paper.
In copying engravings &c. by this method, the lights & shadows are reversed: consequently the effect is wholly altered. But if the picture so obtained is first preserved so as to bear sunshine, it may be afterwards itself employed as an object to be copied: & by means of this second process the lights & shadows are brought back to their original disposition. In this way we have indeed to contend with the imperfections arising from 2 processes instead of one; but I believe this will be found merely a difficulty of manipulation.
I propose to employ this for the purpose more particularly of multiplying at small expense copies of such rare or unique engravings as it would not be worth while to reengrave, from the limited demand for them.
I will now add a few remarks concerning the very singular circumstance which I have before briefly mentioned, viz. that the paper sometimes, although intended to be prepared of the most sensitive quality, turns out on trial to be wholly insensible to light, & incapable of change. The most singular part of this, is the very small difference in the mode of preparation which causes so wide a discrepancy in the results. For instance, a sheet of paper is all prepared at the same time, & with the intention of giving it as much uniformity as possible. And yet, when exposed to sunshine, this paper will exhibit large white spots of very definite outline, where the preparing process has failed; the rest of the paper, where it has succeeded, turning black as rapidly as possible. Sometimes the spots are of a pale tint of cærulean blue, & are surrounded by exceedingly definite outlines of perfect whiteness contrasting very much with the blackness of the part immediately succeeding.
With regard to the theory of this, I am only prepared to state as my opinion at present that it is a case of what is called “unstable equilibrium”. The process followed is such as to produce one of two definite chemical compounds; & when we happen to come near the limit which separates the two cases, it depends upon exceedingly small & often imperceptible circumstances, which of the two compounds shall be formed.
That they are both definite compounds, is of course at present, merely my conjecture: - that they are signally different is evident from their dissimilar properties.
I have thus endeavoured to give a brief outline of some of the peculiarities attending this new process, which I offer to the lovers of Science & Nature.
That it is susceptible of great improvements I have no manner of doubt: but even in its present state I believe it will be found capable of many useful and important applications besides those which I have given a short account in the preceding pages.