Memoire on the Heliograph Joseph Nicéphore Niépce (1833)

The invention which I made and to which I gave the name "heliography" consists in the automatic reproduction, by the action of light, with their gradations of tones from black to white, of the images obtained in the camera obscura.

Basic Conception of This Invention
Light in the state of combination or decomposition reacts chemi­cally on various substances. It is absorbed by them, combines with them, and imparts to them new properties. It augments the natural density of some substances, it even solidifies them and renders them more or less insoluble, according to the duration or intensity of its action. This is, in a few words, the basis of the invention.

First Substance: Preparation
The first substance which I use, the one which makes my pro­cess most successful and which contributes more directly to the production of the effect, is asphaltum, or the so-called bitumin (pitch) of Judea, which is prepared in the following manner.
------I fill half a glass with this pulverized pitch and then pour, drop by drop, lavender oil on it until the pitch will absorb no more and is entirely saturated with it. Then I pour more of this essen­tial oil on it until it stands three "lignes" above the mixture, which is then covered and set in a place of moderate temperature until the oil is saturated by the coloring matter of the pitch. If this varnish is not of the proper consistency, it is allowed to evapo­rate in a dish, protecting it against moisture which would modify and finally disintegrate it. This unfortunate result is particularly to be guarded against in the camera during the cold damp season.
------If a highly polished metal plate plated with silver is coated with a small amount of this cold varnish, using a very soft leather ball, it gives the plate a beautiful red color and spreads over it in a very thin uniform coating. Then the plate is placed on a hot table which is covered over with several layers of paper, from which the moisture previously has been removed, and when the var­nish is no longer tacky, the plate is withdrawn to allow it to cool and permit it to dry completely in a moderate temperature, pro­tected from the influence of the moisture in the air. I must not forget to mention that this precaution is indispensable principally when the varnish is applied. In this case, however, a thin disk is sufficient, in the center of which a short peg is fixed, which is held in the mouth to keep away the moisture of the breath and condense it.
------A plate prepared in this manner may be exposed immediately to the action of light; but even after prolonged exposure nothing indicates that an image really exists because the impression re­mains imperceptible.
------It is therefore a question of developing the picture, and this can only be accomplished by the aid of a solvent.

Preparation of the Developer
Since the developer must be regulated according to the result that is to be obtained, it is difficult to determine the proportions of its composition with exactness; but all things being equal, it is desirable that it be a little weak rather than too strong; that which I prefer consists of one part lavender oil and six parts of white mineral oil or petroleum. The mixture, which at first is quite milky, becomes perfectly clear after two or three days. It can be used several times in succession, losing its solvent prop­erty only when it approaches the saturation point, which is indi­cated by the liquid's becoming turbid and dark in color; but it may be distilled and made as good as before.
------When the plate or varnished tablet is taken out of the camera, it is placed in a white metal dish an inch deep, and longer and wider than the plate, and a plentiful quantity of this developer is poured in it, covering the plate entirely. When the plate is ob­served under an oblique light at a certain angle, the image can be seen to make its appearance, slowly and gradually developing, although still darkened by the oil which, saturated more or less with varnish, flows over it. The plate is then taken from the liquid and placed in a perpendicular position, in order that it may be entirely drained of all developer, after which we proceed to the last operation, which is no less important.

Washing the Plate
A very simple arrangement is required, consisting of a board four feet long and a little wider than the plate. Two strips are nailed on lengthwise, which form a border two inches high. On top is a hinged handle which makes it possible to move the board up and down in order to give the water which is poured on it the required speed. At the bottom is a vessel which receives the liquid flowing off.
------The plate is placed on the inclined board and is kept from sliding off by two small nails or hooks, which, however, must not protrude above the face of the plate. At this time of the year (winter) it must be remembered to use lukewarm water. The water is not to be poured upon the plate itself, but on the board, somewhat above the plate in order to obtain a fall sufficient to carry away the last of the oil adhering to the varnish.
------The picture is now fully developed and appears perfectly and completely defined if the operation has been successful, espe­cially if one has a perfect camera obscura at one's service.

Uses of the Heliographic Process
Since the varnish can be used with equal success on stone, metal, and glass without necessitating any change of procedure, I shall confine myself to the method of application to silvered plates and glass, calling attention once for all to the fact that in printing on copper a little wax, dissolved in lavender oil, may be added to the varnish mixture without damage.
------Until now silvered plates seem to me best suited for the pro­duction of pictures, owing to their white color and their nature. It is certain that after washing, assuming that the image is quite dry, the result is satisfactory. It would be desirable, however, to obtain all gradations of tone from black to white by blackening the plate. I therefore occupied myself with this subject, using at first a solution of potassium sulphide (sulfure de potasse); but if a concentrated solution is used it attacks the varnish, and if diluted with water it turns the metal red. This twofold defect compelled me to give up this medium. The substance which I am now using with greater expectation of success is iodine, which has the property of evaporating at ordinary temperatures. In order to blacken the plate by this method, it is only necessary to place the plate against the inner side of a box that is open at the top and to put a few grains of iodine into a groove cut into the opposite side on the bottom of the box.
------It is then covered with a glass, in order to observe the result, which, although it shows less rapidly, is all the more certain in its effect. The varnish can then be removed by alcohol, and not a trace will remain of the original impression. Since this process is still quite new for me, I confine myself to this simple description until experience has allowed me to collect more precise details.
------Two experiments showing views on glass exposed in the cam­era obscura have furnished me with results which, although still faulty, seem to me quite remarkable, because this mode of appli­cation can be more easily perfected and therefore may become of special interest.
------In one of these experiments the light, which had acted with less intensity, had dissolved the varnish in such a manner that the gradations of tones showed more clearly when viewed by "transmission" (i.e., transmitted light), so that the picture re­produced, up to a certain point, the well-known effects of the diorama.
------In the other experiment, however, where the action of the light was more intense, the lightest parts, which were not af­fected by the developer, remained transparent and the grada­tions of the tones depended entirely and solely on the density of the more or less dark layers of the varnish. If the image is viewed on its varnished side by reflection in a mirror, and held at a certain angle, the effect is greatly enhanced, while if viewed by transmitted light, it appears confused and colorless, and, what is still more remarkable, it seemed to differentiate the separate tones of certain objects. When I considered this curious phenomenon, I believed that I might draw certain conclusions from it, which permitted a connection with Newton's theory of colored rings. It would be sufficient to assume that any prismatic ray, the green for instance, in acting on the substance of the varnish and combining with it, gave it the necessary degree of solubility, so that after the double operation of the developer and the rinsing, the layer which had formed by this method would reflect the green color. Whether this hypothesis is well founded is a matter for future investigation, but the subject seems to me of itself so interesting that it may well deserve further experi­ments and more exact proof.

Although the application of the necessary media described above undoubtedly offers no difficulty, it may happen (when the procedure is not carefully followed) that in the beginning the operation will not turn out well. I believe, therefore, that it is advisable to start in a small way by copying copper engravings in diffused light according to the following very simple method. Varnish the engraving only on the reverse side to make it thoroughly transparent. When the paper is completely dry, place it face down on the coated plate under a glass, the pressure being modified by inclining the plate at an angle of 45 degrees. By this method it is possible to make several experiments in the course of a day, using two engravings properly prepared and four small silvered plates. This can be done even in overcast weather, pro­viding that the workroom is protected against cold and especially against moisture, which, I repeat, deteriorates the varnish to such an extent that it will float off in layers when the plate is immersed in the developer. It is for this reason that I ceased using the camera obscura during the inclement season. If the experiments which I have described are continued, one will soon be well able to carry out the details of the whole process.
------In the matter of applying the varnish, I must call attention again to the fact that it can be used only in a consistency which is thick enough to form a compact yet thin coating, in order that it might resist better the action of the developer and become at the same time more sensitive to the action of light.
------In respect to the use of iodine for blackening the images pro­duced on silvered plates, as well as regarding the acid for etching the copper, it is essential that the varnish after washing be used exactly as described above in the second experiment on glass; because it becomes thus less permeable, both in acid and under the iodine vapors, particularly in those parts where it has re­tained full transparency, for only under these conditions can one hope, even with the best apparatus, to obtain a completely suc­cessful result.

When the varnished plate is removed for drying, it must be care­fully protected not only from moisture but also from any expo­sure to light. In speaking of my experiments in diffused light, I have not mentioned any of these kinds of experiment on glass. I add this in order not to omit a specific improvement which re­lates to them. It consists simply in placing a piece of black paper under the glass and in putting between the metal plate on its coated side and the copper engraving a border of cardboard on which the engraving has been tightly stretched and glued. This arrangement has the effect of making the image appear much more vivid than on a white background, which helps to acceler­ate the action, and, furthermore, of avoiding damaging the var­nish by rubbing it against the copper engraving as in the other method, a mishap which is very hard to prevent in warm weather, even when the coating is quite dry. This disadvantage is counterbalanced, however, by the advantages offered by the experiments with silvered plates, which withstand the action of the washing better, while it is rare that the images on glass are not more or less damaged by this operation, for the simple reason that the varnish can adhere less easily to the glass, owing to its nature and its smooth surface. It would be necessary, therefore, in order to overcome this disadvantage, to improve the varnish by making it more sticky, and I believe that I have succeeded in doing this at least in so far as I may be permitted to pass judgment on this matter, although the experiments are still new and not numerous enough.
------This new varnish is composed of a solution of bitumen of Judea in Dippel's animal oil, which is allowed to condense at the ordinary temperature of air to the degree of consistency re­quired. This varnish is more greasy, tougher, and more strongly colored than the other, and it can be exposed to light as soon as the plate is coated, because it seems to solidify more rapidly, owing to the great volatility of the animal oil which causes it to dry more rapidly.