BRITISH JOURNAL OF PHOTOGRAPHY. August 15, 1863, p.326
IF it were possible, we should willingly postpone the publication in order to make our investigation more complete; but the near approach of the photographic campaign, and the desire to avoid the reproach of incompleteness in our directions which might be fairly brought against us if the results were not obtained which ought to follow the application of our theory, will not, allow us to wait longer.
We have said that it was possible to continue out of the camera the action of light on the image produced, in such a manner as to increase its intensity.
When it is desired not to light the whole of the negative, those parts which it is desired to retain in their primitive state must be protected by screens.
How is this result to be obtained? That is what we now proceed to show. But it should be at once understood that we do not here propose to give an artistic lesson. Art is not the result of chemical reactions or manipulations: it consists in the choice of effects. We shall show how they are obtained, not where they should be produced.
Some parts of a subject have to be sacrificed in order to bring others out into bolder relief and with greater effect than with an ordinary method of illumination, as in the magic paintings of Rembrandt; or ask of Nature a lively and spirited style of illumination, as in the Spanish school. Photography will be able, from this time; forth, to produce these two results with the same material
Let its observe at first, that there are certain kinds of negatives which we prefer to others. Thus those of a medium degree of intensity and full of detail--that is to say, rather weak than strong--are those with which we are; best satisfied. The following is the reason:--their deficient intensity causes them to be rejected in practice, because, in printing, they yield proofs which are too deep and without brilliancy in the high lights--a condition of things which is very favourable for our purpose. All those parts which we desire to keep in relative obscurity are obtained in that condition by using the negative in its ordinary, condition; that is to say, that we protect those parts from the action of light while we expose the others.
Nothing can be more simple; than the means necessary to effect this object. With a small brush, and some opaque colour well ground in oil or varnish, we trace out, as correctly as possible, on the reverse of the cliché--that is to say, on the side not covered with collodion--the outline of those parts which it is desired to protect from the action of light; then this being determined--a matter of great facility by reason of the transparency of the negative--we cover these parts with a film of opaque colour.
In this state we expose to the sun the painted side of our negative.
The luminous rays pass through those parts not protected by colour, and it is on these places only that they exercise their colouring action.
It will be obvious that it is quite possible to arrive at great perfection in defining the outline of those parts which it is desired to protect and those which it is desired to expose.
A sufficient amount of sharpness almost is obtained without the slightest precautions. Further, an amount of sharpness equal to that produced by an engraver's tool may be obtained if the negative be exposed on a moveable framework, moving in the meridian, and according to the elevation of the sun, in order that the rays may traverse the thickness of the glass in a perpendicular direction.
The mass of high lights being obtained, the working of the reserves may be proceeded with at pleasure. All those parts of the negative which are now considered sufficiently exposed must be covered with colour, in order that those only may now be exposed to which it is deemed expedient to impart greater depth of colour. It will be seen that in this way we may, vary, ad infinitum, the luminous effects, since we. can graduate at discretion the intensity of the light.
When the desired effect has been obtained, the colour with which the back of the negative has been covered is removed, and a little practice will enable the operator soon to appreciate the value of the lights thus produced.
It is scarcely necessary to mention that if the contemplated effect has not been fully attained, nothing is more simple than to re-expose those parts which require to be more deeply printed--an operation which in some respects resembles that of the engraver, who continues his labours until. he has obtained what he desires.
The effects due to luminous action being finished, we have at our disposal two methods-either of which may be adopted, according to circumstances--for the purpose of de-iodising or fixing.
If the negative under treatment be strongly coloured, and possess that degree of intensity necessary to give brilliant positive proofs. it is only necessary to introduce it into a bath of hyposulphite of soda. The effect of the re-agent may be followed by the eye. In dissolving the iodide it simultaneously removes the pearly shade of the collodion film, which then becomes completely clear and transparent. If, on the contrary, either from insufficient exposure or want of energy in the light the negative be regarded as insufficiently intense, it will be necessary to submit it to the action of chloride of gold according to the very valuable method suggested by M.. Fizeau for Daguerreotype pictures on silver plates.
Let us recapitulate this method in a few words, to avoid trouble to those of our reader;, who leave not practised the Daguerreotype:--The plate, being perfectly levelled on a levelling stand is flooded with a solution of Sel d'or de Fordos et Gelis (hyposulphite of soda and gold) as deep as possible. The image thus covered is uniformly warmed by means of a spirit lamp, under the orison of which the gold is precipitated and the image is seen to change colour.
We propose to replace the Daguerreotype plate by our glass negative, and let it undergo the same treatment. Whether it has previously been grey or brown, we shall see it assume a deep black tint when viewed by transmitted light, and bronze green by reflected light. The opacity naturally results from the quantity of gold precipitated; we therefore warm it well in order to have an abundant precipitate, and we use a very strong solution of gold (containing one per cent).
This process, as will be seen, constitutes in itself an excellent method of intensifying; for, far from thickening the high lights and fogging the shadows--as too often happens when intensifying by means of a nitrate of silver bath--it gives to the high lights great solidity without taking away their limpidity, and instead of fogging the high lights it clears them, so to speak, by beginning the solution of the iodide of silver in the film.
But for the special case upon which we are now engaged it has an advantage which does not appertain to any ordinary means of intensifying, inasmuch as it induces a much more energetic action upon those parts where the light has twice acted than upon those parts which have been preserved. Thus, in our own practice, we are almost tempted to submit all our negatives to the action of sel d'or before de-iodising.
After cooling, the excess is collected to serve (after the addition of see d'or to keep up its strength) again. The negative is then, treated with hyposulphite of soda to effect the removal of the iodide: it is then freely washed with water and dried.
We have said that when we had to deal with a weak negative, but full of detail, as produced by certain reducing agents, especially sulphate of iron, we could. convert into shaded portions for the general effect of the picture those portions of the negative which are exposed to a second action of light. It is possible that by this simple means we should have shadows enough. When, however, this is not the case, and it becomes necessary, for effect, to sacrifice more of certain parts, the following is the method to adopt to obtain this result:--
Trace upon paper against glass the outline of those parts which it is desired to put into shadow; cut this out in cardboard or thick paper, in such a manner that this cardboard being applied to the proof protects one part, while the other is being exposed.* Prepare then a flat dish, which, being placed horizontally, should leave some iodine spread over the bottom, over which the negative, covered by its screen, should be placed. The vapours of iodine attach themselves to those parts of the image which remain uncovered, and form with the metal an iodide of silver. The film is more or less thick, according as the exposure has been more or less prolonged.
Iodide of silver being soluble in hyposulphite of soda and its analogues, it is only necessary to pass the negative rapidly through a solution thereof in order to remove all the iodide formed. By these means the shadows may be increased to any desired extent, since it may be carried as far as the complete conversion of all the metal into iodide.
It is hardly possible to say at what point the iodising should be stopped, as experience alone can teach this; the operator will, however, be able to form some judgement from the colour assumed by the negative.
It will, of course, be understood that it is always best to keep within the extreme effect, since one is always free to renew the operation; as the negative, after iodising, drying, and washing, tray be submitted to the action of iodine until the image is completely absorbed.
Let us conclude by an observation which is not without its importance. It is, that the operation of iodising is attended with less danger after the negative has been passed through chloride of gold, as we showed above. The film of gold which covers the silver moderates the action. of the iodine, and thus allows it to be conducted with greater safety to the point where it is desirable it should be arrested.
* Tracing the outline on glass may be dispensed with if a positive print be taken, pasted on cardboard, and cut out. In this manner a more exact outline is obtained than by tracing.