THE BRITISH JOURNAL OF PHOTOGRAPHY. August 1, 1863
PART I.-INTRODUCTION-THEORY.
"From The Bulletin de la Society Française de Photographie
THE subordinate influence exercised by art alone, in the production of a photographic negative is very generally admitted. The choice of subject, the means of illumination, and the pose fall infinitely short of that which it is desired to attain, when it becomes a question of, reproducing nature; far short as it is, however, it is easy to recognise it in the productions of those operators who possess artistic skill, while its absence is equally obvious in the works of those who owe their success chiefly to science and facility of manipulation.
Furthermore, with what regret do we not see, in the midst of the great progress which science every day effects in favour of photography, art remain in the same relation thereto that it did at the beginning--powerless to modify by taste or feeling the pictures produced by the Daguerrean machine? How great would be the change which photography would experience if the operator were left free to regard the image traced by the luminous ray in the camera obscura as a faithful and beautiful sketch waiting only artistic inspiration to complete it! If it were so it would be like those rare proofs which, while carefully reproducing the work of the master, also bear the impress of the individual sentiment of the engraver, and thus sometimes produce an effect superior to the original. The problem to be resolved, therefore, is so to guide the marvellous but unintelligent powers of the camera ill the production of the image as to give to the photographer the means of continuing and modifying at pleasure its aspect and significance--to substitute, so to speak, his work for that of the camera, making use of the same chemical means of production.
It will not be accomplished by the use of galvanic action in the manner which we were the first to describe in 18511 and which consists in precipitating, by means of a metallic bath, a fresh quantity of metal to strengthen the already too feeble image. This process, in use every day, cannot be practised successfully except all over the surface of the mould. Its results will not resolve our problem, because it does not modify the effects of the image, it only changes the course. Neither will the desired result be obtained by that method of retouching which a skilful hand is sometimes obliged to give to a negative. What pencil would be sufficiently delicate to associate its labours with those: of the luminous ray? Moreover the result which it is desired to obtain is the strengthening this or that part of the negative at pleasure.
The artistic achievement of which we are now speaking cannot be attained either by the pencil or the pallette; it must remain purely chemical, in order not to alter the outline and to allow the image to retain all its homogeneity and delicacy of execution. It is necessary to find an agent sufficiently subtle to rival light, or, better still, to render light itself subservient to our purpose. The problem to he resolved, therefore, may be expressed in the following words:
Can. light be induced to continue and modify, according to the taste of the operator, the image formed in the camera?
When a film of iodised collodion is immersed in a solution of nitrate of silver, an interchange of bases takes place, resulting in the formation of iodide of silver with excess of nitrate. This constitutes what is called in practice the sensitive film.
If the sensitive film be exposed in the camera, all the more or less luminous points act thereon in proportion to their intensity, and at the end of an ordinary exposure there is a complete though latent image formed. It might be imagined that this image would be visible at least by transmitted light, but it is not so, at all events to the naked eye. It is necessary in order to bring out the latent image that it should be submitted to the action of one of the three re-agents in ordinary use--gallic acid, pyrogallic acid, or sulphate of iron. It, then becomes manifest in the form of a metallic precipitate, weak in those parts, where the light has least acted, and abundant where the action has been more intense. It is from this varying degree of transparency and opacity in those parts of the image, arises that beautiful gradation of light, and shade which makes the charm of successful pictures.
The amount of precipitate produced by the reducing agent, then, on the impressed portions of the film is in proportion to the intensity of the light's action. If the image is deficient in opacity, on account of the weakness of the precipitate, it is because the luminous action has been insufficient.
It therefore becomes necessary to submit the negative to a second exposure in the camera. But this result, good as it may seem in theory, has not the least practical value. It is impossible, with most rare exceptions, to obtain a second exposure under identically the same conditions; and, even if it were so, if it were ever so easy, we should be only doing the same thing over again, and that of course is not what we aim at.
It would be altogether different if it were possible to isolate any portion of the subject--or in other words, if we could preserve the photogenic properties of the various objects of which it is composed, and at the same time render insensible to a new impression the metal which, not having been at first acted on, did not aid in the formation of the image.
In order to arrive at .a. satisfactory solution of this interesting problem, let us first study the conditions of the formation of ail image on a collodion film.
Contrary to what occurs in a paper negative, where the image is deeply impressed in the very pores of the paper, the image on a collodion negative is formed by the reduction of silver precipitated in the state of metallic powder, without adhering to the collodion film, from which it may be removed by the slightest touch without leaving any trace of the image.
It is evident that this reduction takes place only with the free silver adhering to the plate after the formation of the iodide of silver.
In fact, if by washing freely with water the greater part of the adherent nitrate of silver which in ordinary practice remains on the film during exposure be removed, the developer will give only a comparatively feeble image. If after this a solution of nitrate of silver be poured over the plate the image appropriates the silver of which it was deficient, and soon becomes strongly visible. Now, if instead of confining ourselves to washing the film simply with water it be covered with a reducing salt, which, combining; with the excess of nitrate, carries it away in the washing, and if it be then exposed in the camera and the action of the developer renewed to any extent, no trace of the image will become visible, although the nucleus is certainly there, since it becomes evident on the addition of the nitrate of silver.
Let us well observe the difference in the two results:---In the first instance, washing with plenty of water and production of a feeble image; in the second, washing followed by immersion in a solution of reducing salt, and complete absence of image.
It is well established
1. That no image is produced without the presence of an excess of nitrate on the iodide of silver.
2. That if water alone is insufficient to remove all the excess of nitrate, it is not so with the reducing agent which combines there-with and carries it away in the washing, which should always follow immersion in the reducing bath.
As it is demonstrable that the reducing agent removes from the film. of iodide all the salts of silver in excess, and that this iodide, while fully sensitive to light, does not manifest any sensible alteration until it is placed, after exposure, in contact with a fresh quantity of silver, we have attained the solution of our problem.
In fact, on the one hand--as we leave stated above--the image is only imperfectly produced under the action of the developer when the impression by light has been feeble, and it is sufficient, in order to intensify the image, to prolong or renew the exposure; and if, on the other hand, the image, produced by silver exposed to light, and precipitated by the reducing agent, preserves its photogenic properties, when it has not been destroyed by the de-ioduration of the collodion by means of hyposulphite of soda or of cyanide of potassium, it will only be necessary for the operator to expose, develope, wash, and re-take the negative, which, in this state, should be kept in the dark until it be desired to modify the design obtained.
If it be desired, certain parts may be cleared to give them greater importance or make them more emphatic, as it is necessary in this case to increase the opacity of the negative. The operator should expose it to daylight, taking care to protect from the action of light, by screens more or less directly interposed, those parts which it is desired to retain in their primitive condition.
He can--according to his taste-interrupt, displace, or guide this work of the light quite slowly enough to be followed lay the eye.
When the desired effect is produced, all that is necessary is to finally stop the action by plunging the negative in hyposulphite cyanide to remove therefrom its photogenic properties.
In a picture composed of light and shadow it is not sufficient, to obtain the desired artistic result, to have at command a luminous pencil which can be directed at discretion to any part of the proof: it is necessary to be able to weaken the too vivid lights and produce shadows.
Pure or complete shadow being the negation of an image, to destroy the image or to lessen it should be the object aimed at--only to destroy without altering, to lessen without spoiling, that is the question.
A knowledge of the chemical composition of the image will be found to aid us in solving this difficulty. It is already admitted that the image is produced by a precipitate of silver, and its value in producing light and shade depends on its opacity. Wherever we destroy the whole or part of the opacity, we produce shadows or weaken the high lights. It is only necessary, in order to produce this result, to dissolve chemically the whole or part of the metallic film. The means of obtaining this last result is very simple.
It is known that iodine forms with silver an iodide of silver soluble in certain re-agents.
On submitting to the action of iodine vapour those parts of the picture the density of which it is desired to diminish, a film of iodide of silver is formed proportioned to the iodine supplied. It is possible, therefore, to convert a portion of the thickness of the image into iodide at pleasure; this being done, by passing the negative rapidly through a bath of hyposulphite of soda, all the iodide is removed without danger of dissolving the silver not converted into iodide. This operation may be repeated as often as is necessary to obtain the desired result.
It will be understood that, acting with vapours which do not attach themselves except to the surface of the image, it remains in this condition with lessening of the metallic film only in those parts where for the desired artistic effect it has been deemed necessary to diminish the opacity.
The proof2 attached to this paper is given as an illustration of the two actions we desire to describe. The lights and shadows have been produced at pleasure upon the same image without any artistic intention, and with the sole object of furnishing a demonstration of the facts announced.
Up to this point we have referred only to the applications of our new method of clearing up negatives which would be made when artistic taste and sentiment call be drawn upon. We should say a few words to those practitioners whose ambition rises no higher than producing the best result which can be obtained in the camera direct.
For them the question which should take precedence of all others is how to obtain the best negative, by the shortest possible exposure. We might here enter into the infinite number of details on the nature of the iodides, their relative proportions in collodion, the acidulation or neutrality of the silver bath, all the causes of accelerating or retarding photogenic operations; but that would draw us away from our subject. Let us simply endeavour to ascertain what advantage photography, as practised daily, may derive from the application of our method of intensifying.
It is well known that, in the use of the three ordinary developers, the negatives which are most complete as to details are the finest in modelling, and generally most harmonious are those which are produced in a moderate light, and developed by sulphate of iron; but they are too transparent to give good prints. The reduced silver constituting the image allows the light to pass too freely, and all the details which exist in the negative in such perfection are in a great measure lost in the positive proof:
To render this sort of negatives available, it is necessary to have recourse to what is termed in practice re-developing. All the processes of this kind in daily use, and they are numerous, rest on the principle previously explained-a galvanic action precipitating on the already reduced metal a new film of the same material. Except in those cases where these reductions are too weak we obtain, no matter by what means, the intensifying of the image; but let us see at what cost.
At first, if, as is generally the case, the. subject has light parts which appear rapidly in developing, like the head, hands and linen in a portrait--buildings and foreground as compared with trees in a landscape--the details possessed by these parts become clogged in the operation of re-developing and for a little detail gained in the shadows we lose sometimes every trace of detail in the high light.
But this is not all the danger. Under the galvanic action which is set up by means of a metallic solution and a reducing agent the collodion film becomes softened, it puckers, splits, and sometimes in the necessary washings it becomes completely detached from the glass.
We are liable, therefore, in redeveloping, to lose the delicacy of the proof, and also to risk the losing of the negative altogether.
Again, if it were possible, to confine the redeveloping only to those parts which required it; but the repeated failures which attend such efforts to circumscribe the action demonstrate only too forcibly how defective are the means hitherto employed for this purpose.
We will not insist on the advantages to be gained by the adoption of the partial method of developing which we have indicated above; but we would say to those who, not desiring to change the method of producing their negatives, wish, nevertheless, to give them more brilliancy that, in applying our method of redeveloping, there is no fear of softening the film or any of its accompanying evils. The prolonged exposure to light (one or two hours in summer and an entire day in winter) to which we submit our negatives induces a hardening of the collodion; moreover, we operate slowly in the dry state and in full daylight, which admits of watching the progress of the operation and stopping it in time. We are not obliged to go beyond the mark, as is often the case in the wet process by artificial light.
OF all preparations used in photography for the production of negative proofs, that which offers the greatest advantage is collodion. The only drawback is, that its employment in the wet state necessitates such a cumbrous baggage that the efforts of photographic travellers are directed to the utilisation of dry collodion, which it is not necessary to prepare on the spot, and need not be used at the time of sensitising, as is unavoidable when the film is in the wet; state.
Unfortunately the use of dry collodion is not without its difficulties. To the introduction of foreign bodies such as molasses, to weaken the corrosive action of the nitrate of silver on the collodion, is doubtless attributable the numerous disturbances which supervene in the after operations, and render the final result often very unsatisfactory.
Our own opinion is that it is to the existence of free silver on the surface of the sensitive film (erroneously regarded as necessary to the formation of the image), that the frequent changes so inconvenient to operators should be attributed. It is certain that in depriving the iodised collodion of this free silver we do not destroy its photogenic properties.
In view of this principle we propose the following method for working the dry collodion:
The collodion film being prepared and sensitised in the ordinary manner, let it be immediately treated with a solution of sulphate of iron to carry off all the free silver remaining on the surface of the plate. Wash the plate completely, and allow it to dry in the dark. In this state expose as soon as convenient in the camera, or postpone the exposure according to circumstances. The exposure being finished, the development may be postponed also; but, when the time for development does arrive, the negative must again be treated with sulphate of iron, and developed thereby with the addition of a weak solution of nitrate of silver (about 3 per cent;).
It may be fairly assumed that such a simple process would work out satisfactorily, and of its great convenience on a journey there can be no doubt. We propose to devote further attention to it when more extended experience shall have confirmed our experiments.
[The Second Part of the above paper, consisting of manipulatory details, will be given in our next.-ED.]
1. Traite de Photographie sur Papier. Paris, Rout.1851
2. In Le Bulletin a photographic illustration is given proving most unequivocally the intervention of the influence described, the absence of artistic intention demonstrating the impossibility of its having occurred in the camera. ED.