The discovery which I have made, and to which I give the name Heliography, consists in reproducing spontaneously by the action of light, with gradations of tints from black to white,* the images received by the Camera Obscura.
Fundamental Principle of the Discovery.
Light, in its state of composition and decomposition, acts chemically upon bodies. It is absorbed, it combines with them, and communicates to them new properties. Thus it augments the natural consistency of some of these bodies: it solidifies them even, and renders them more or less insoluble according to the duration or intensity of its action. Such, in a few words, is the principle of the discovery.
The substance or primary matter I employ,—that which has succeeded best with me, and which concurs most immediately to produce the effect isAsphaltum or Bitumen of Judea, prepared in the following manner. I fill a wine-glass about half with this pulverised bitumen. I pour upon it drop by drop the essential oil of lavender till the bitumen can absorb no more, and till it be completely saturated. I afterwards add as much more of the essential oil as causes the whole to stand about three lines above the mixture, which is then covered and submitted to a gentle heat until the whole essential oil be saturated with the colouring matter of the bitumen. If this varnish should not yet possess the requisite consistency, it is to be allowed to evaporate atmospherically in a shallow dish, care being taken to protect it from moisture, by which it is injured, and finally decomposed. If in winter, or during rainy weather, the precaution is doubly necessary.
 The clearest tint which is obtained by this process is not white. — Louis Daguerre.
A small quantity of this varnish applied cold, with a light roll of very soft skin, to a highly polished tablet of plated silver, will impart to it a fine vermilion colour, and will cover it with a very thin and equal coating; the plate is afterwards to be placed upon heated iron, which is wrapped round with several folds of paper, whence by this means all the moisture has previously been expelled. When the varnish has ceased to simmer, the plate is withdrawn, and left to cool and dry in a gentle temperature, secured against contact with a damp atmosphere. I ought not to omit mentioning that it is principally in applying the varnish that this last precaution is indispensable. In this part of the operation, a light circle of metal, with a handle in the centre, should be held before the mouth in order to condense the moisture of the respiration.
The plate thus prepared may be immediately submitted in the focus of the camera to the impressions of the luminous fluid. But even, after having been thus exposed, a length of time sufficient for receiving the impressions of external objects, nothing is externally apparent to show that these impressions exist. The forms of the future picture remain still invisible.f The next operation then is to disengage the shrouded imagery, and this is accomplished by a solvent.
 It is impossible by any such means to spread a coating sufficiently equal to obtain in the camera that delicacy required by the modifications of light. If the image were altogether invisible, there could be no result. M. Niepce means that the impression is extremely feeble, but if there be no appearance of action at all, the operation has not succeeded. — Louis Daguerre.
Of the Solvent, and Manner of its Preparation.
As the solvent must be adapted to the purposes for which it is designed, the task is difficult to fix with certainty the proportions of its components; but in all cases it is better that it be too weak than too strong. That which I employ in preference, is composed of one part, not by weight but volume, of essential oil of lavender poured upon ten parts, by measure also, of oil of white petroleum. The mixture which is first of a milky consistency, becomes perfectly clear in two or three days. This compound will act several times in succession. It loses its dissolving power only when it approaches the point of saturation; this state is readily distinguished by an opaque appearance and dark brown color.
 These two extremes have their respective inconveniences. In the former he image does not re-appear with sufficient brilliancy; in the latter it is completely destroyed.— Louis Daguerre
The plate or tablet varnished as described, and exposed as directed, having been withdrawn from the camera, a vessel of tinned iron somewhat larger than it, and about an inch deep, is previously prepared and filled with the solvent to a depth sufficient to cover the plate. Into this liquid the tablet is plunged, and the operator, observing it by reflected light, begins to perceive the images of the objects to which it had been exposed, gradually unfolding their forms, though still veiled by the supernatant fluid continually becoming darker from saturation with varnish. The plate is then lifted out and held in a vertical position till as much as possible of the solvent has been allowed to drop away. When the dropping has ceased, we proceed to the last and not least important operation.
Washing.—Manner of Procedure. A very simple apparatus answers for this operation, namely, a board about four feet long, and somewhat broader than the tablet. Along each side of this board runs a ledge or border projecting two inches above its surface. It is fixed to a support by hinges at its upper extremity, in such a manner as permits its angle of inclination to be varied at pleasure, that the water thrown upon it may run off with the requisite velocity. The lower end rests upon the vessel intended to receive the water as it flows down.
The tablet is carefully placed upon the board thus inclined, and is prevented from slipping down by two little blocks, which ought not to exceed the thickness of plate, that there may be no ripple in the descending stream. Tepid water should be used in a cold day. The water must by no means be poured directly upon the plate, but above it on the board, so that descending in a stream it may clear away all the remaining solvent that may yet adhere to the varnish.
Now, at length, the picture is completely disengaged, and if the different operations have been carefully performed, the outlines will be found to possess great neatness, especially if the images have been received in a camera with achromatic lenses.
 Here M. Niepce seems to speak hypothetically; for experience proves that the achromatic camera, though it gives greater purity to the images, does not impart that superior neatness and sharpness to the outlines which were anticipated from its operation.— Louis Daguerre.
When the plate is removed to be dried, which must be done with great care, by a gentle evaporation, it must be kept protected from humidity, and covered up from the action of light.
Applications of the Heliographic Processes.
The varnish employed may be applied indifferently to stone, metal, and glass, without any change in the manipulation. I shall speak, however, only of its application to plated silver and glass; as to impressions on copper, I may just state that a small quantity of wax dissolved in essential oil of lavender, may in this case be added with advantage to the varnish already described.
 Here M. Niepce teems to speak hypothetically; for experience proves that the achromatic camera, though it gives greater purity to the images, does not impart that superior neatness and sharpness to the outlines which were anticipated from its operation.— Louis Daguerre.
Of all substances hitherto tried, silver plated upon copper appears to me to be the best adapted for reproducing images, by reason of its whiteness and structure. One thing is certain, that after the washing, provided the impression has been well dried, the result obtained is already satisfactory. It were, however, to be desired that, by blackening the plate, we could obtain all the gradations of tones from black to white. I have, therefore, turned my attention to this subject, and employed at first liquid sulphate of potassa. But, when concentrated, it attacks the varnish; and, if reduced with water, it only reddens the metal. This twofold defect obliged me to give it up. The substance which I now employ is iodine,t which possesses the property of evaporating at the temperature of the atmosphere. In order to blacken the plate by this process, we have only to place it upright against one of the sides of a box, open above, and place some grains of iodine in a little groove cut in the bottom, in the direction  of the opposite side. The box is then covered with a glass, to judge of the slow but certain effect. The varnish may then be removed by spirit of wine, and there no longer remains any trace of the original impression. As this process is still quite new to me, I confine my remarks to this simple explanation, waiting till experience shall enable me to enter upon more circumstantial details.
 It is necessary to remark that the "impressions on copper," of which M. Niepce here speaks, have reference solely to copies of engravings placed above the prepared plate, and thus exposed to the full action of the sun's light. The application of the wax, as mentioned by him, would have neutralized the effect of the decomposition of the bitumen in the camera into which the light enters, much weakened. But the presence of this wax was not an obstacle in copying engravings exposed three or four hours to the direct rays of the sun.— Louis Daguerre.
[Daguerre's own method, as the reader will hereafter understand, depends on this decomposition of iodine. This is the great distinction between the old and new processes—between Niepce's and Daguerre's systems—in a word, between the approximation and the real principle.— Translator.]
Two experiments in landscape upon glass, by means of the camera, gave me results which, though very imperfect, appear deserving of notice here, because this species of application may be brought more easily to perfection, and become in the sequel a most interesting department ofHeliography. In one of these trials, the light having acted with less intensity, removed the varnish in a way which exhibited a more marked gradation of tone, so that, seen by transmitted rays, the landscape produced, to a certain extent, the well-known effects of the Diorama.
 M. Daguerre does not see what relation can exist between the effect here described by M. Niepce and the pictures of the Diorama.
[The reader will find in M. Daguerre's own description of Dioramic painting, that much of its surprising effect depends upon the transparency of the ground; both reflected and transmitted illumination throws light up, or, as the case may be, deadens the effect of these pictures. A medium, or (if the expression be allowed) canvas of glass, with a picture sketched in varnishes of different thickness, and, consequently, different transparency, as in the case of Niepce's experiment, would produce the same result, though in a less degree, owing to the absence of color.—Translator.]
In the second trial, on the contrary, in which the action of the luminous fluid had been more intense, the parts acted upon by the strongest lights, not having been attacked by the solvent, remain transparent, and the difference of tones results entirely from the relative thickness of the coatings of varnish.
If this landscape be viewed by reflection in a mirror, on the varnished side, and at a certain angle, the effect is striking, while, seen by transmission, it presents only confused and shapeless imagery; but what is really surprising, in this position the mimic tracery seems to affect the local colors of certain objects.
In reflecting upon this remarkable fact, I have sometimes thought that consequences might thence be deduced connected with Newton's theory of colored rings. It is sufficient further to suppose that any prismatic ray—the green ray for instance—in acting upon the substance of the varnish and combining with it, imparts the degree of solubility, which, after the double operation of the solvent and the washing, is required to reflect the green color. But it remains for experiment, which alone can decide, to determine what truth there may be in this theory: but the circumstance appears to me sufficiently interesting to excite new researches, and to merit more profound enquiry.
Observations and Additions.
Although doubtless there be nothing difficult in the processes just described, yet it must not be expected that one will succeed at once. I am of opinion then that it is better to begin by copying engravings in full daylight, before attempting the camera, and by a very simple procedure, as follows:—
The engraving must be varnished, but on the back so as to render it quite transparent. When perfectly dry it is applied face upwards to a tablet prepared as already directed  and made perfectly smooth and flat by means of a plate of glass, and to diminish the pressure the apparatus is inclined at an angle of 45°. With two engravings thus prepared, and four small tablets plated on both sides, one may make several experiments in one day, even in the winter or in dull weather, provided the apparatus be protected from the cold and moisture, which latter, I repeat, so injures the varnish as to detach it in flakes from the plate when plunged into the solvent. This prevents me from making use of the camera during the winter season. By repeating these trials one will soon become quite expert in all the processes of manipulation.
 M. Daguerre has often remarked this effect of color, but could never regard it as the effect of the colored rays in the camera.
[M. Daguerre is right; extremely thin laminae of all transparent substances, as a coating of varnish upon glass, give forth prismatic rays, on an optical principle obvious enough. The reader will understand the importance attached to these trifling indications of color, on reflecting that to obtain color as well as form, is the great desideratum wanted to perfect the photographic process. —Translator.]
Respecting the manner of applying the varnish, I observe that it ought to be of such consistence as will form a compact washing, and should be laid on as thin as possible. The former quality enables it better to resist the solvent; the latter renders it more sensible to the solar action. With regard to the iodine for blackening heliographic sketches, just as in the case of acid for engraving on copper, it is essential that the varnish after the washing be as described in the second trial on glass, because the lights resist the solvent in the one case, and the vapors of the iodine in the other, that is, the resistance is greatest in those places which are most transparent; and it is only on this condition, even with the best apparatus, that one can hope completely to succeed. 
 The sketch which gave rise to this remark was for a length of time exposed to the action of light in the camera, and although M. Niepce here speaks of iodine to blacken, and acid to engrave, supposing the impression to be on copper, these two operations could have brought out no gradation of tints. In reality the images having been obtained by a greater or less degree of tenuity in the coatings of varnish, according as it had been more or less acted upon by the light, it is impossible that acid could act upon metal in a manner analogous. Neither did M. Niepce ever make an engraving from a sketch obtained in the camera.— Louis Daguerre.
In speaking of experiments made in open day, and without the camera, I have said nothing of my experiments of this kind on glass. I shall here supply this omission, that I may not forget an improvement peculiar to this description of heliography. It consists simply in placing a black paper under the glass, and interposing a frame of pasteboard between the varnished surface of the glass plate and the engraving, which is previously to be well fixed and stretched upon the frame. By this arrangement heliographic impressions appear much more lively than upon a white ground, and the effect is more speedily produced. Secondly, the varnish is not liable to be injured by coming into immediate contact with the back of the engraving, a disadvantage, which otherwise in the case of the metal plate it is difficult to avoid, especially in warm weather, the most favorable season for these experiments.
But this inconvenience is amply compensated by the superior strength which drawings upon silver plate possess for enduring the process of washing; while it is rare that this operation does not more or less damage drawings upon glass, a substance to which, from its high polish, the varnish more weakly adheres. It becomes an object, then, how to remedy this defect by giving more unctuosity to the varnish, and this, I believe, I have accomplished, at least so far as I can venture to infer from trials still too recent and too few for perfect conviction. This new varnish is composed of Bitumen from Judea dissolved in animal oil of Dippel, which is allowed to evaporate by atmospheric temperature till of the required consistency. This composition is more unctuous, of greater tenacity, and higher color than the other, and after being applied it can immediately be submitted to the action of the light, which appears to render it solid more quickly from the greater volatility of the animal oil. 
 But this very property diminishes still farther the resources of the process as respects the lights of the drawings thus obtained.— Louis Daguerre.
I have retained a copy of this paper, and transmitted the original to M. Daguerre, 5th December, 1829.
(Signed) J. N. NIEPCE.History and practice of photogenic drawing - by Louis DaguerreTechnologyInvention