" For this purpose I must remark, in the first place, that if a piece of black gauze or crape is the object selected for representation, it produces an engraving of itself which is marvellously accurate. But when two folds of the gauze are laid across each other obliquely, then the resulting engraving requires a lens in order to separate from each other and distinguish clearly the lines belonging to the two portions of the gauze. Now, if this engraving is printed off, the result offers to an eye at a moderate distance the appearance of a uniform shading. Now, I avail myself of this circumstance to modify my original process as follows :—suppose the object to be the opaque leaf of a plant, of irregular outline, first, I cover the prepared plate with two oblique folds of black crape or gauze, and place it in the sunshine for two or three minutes. The effect of this is to cover the plate with a complicated image of lines passing in all directions. Then the leaf is substituted for the crape, and the plate is replaced in the sunshine for two or three minutes more. The leaf being then removed from the plate, it will be seen that the sun has obliterated all the lines that were visible on the parts of the plate exterior to the leaf, converting all those parts to a uniform brown. But the image of the leaf itself is still covered with a network of innumerable lines. Now, let this be etched in the way already described, and let the resulting etching be printed off. The result is an engraving of the leaf, which, when beheld by the eye at a certain distance, appears uniformly shaded, but when examined closely, is found to be covered with lines very much resembling those produced by an engraver's tool, so much so that even a practical engraver would probably be deceived by the appearance. This crape arrangement I call a photographic veil; and as I think it likely that the idea will prove useful, I will make a few more remarks upon it. It is clear that an arrangement composed of two thicknesses of ordinary crape or gauze is but a rude attempt at a photographic veil. To realize the practical utility that may result from the idea, supposing it to be borne out by farther experience, it would be proper to fabricate a much finer material, and to employ five or six thicknesses of it, or else to cover a sheet of glass in any convenient manner with an innumerable quantity of fine lines, or else with dots and specks, which must be opaque and distinct from each other. The result of practically employing such a method, supposing always that it answers in practice, as I think it probably will, would be an etching apparently uniform, but really consisting of separate small portions, in consequence of which it would hold the ink much better, and other obvious advantages would also be obtained. Another mode of accomplishing the same object is to cover the plate originally with an aqua-tint ground. But then a fresh one would be required for every plate, whereas a single veil would serve for any number of plates in succession. Experience alone can decide between these different methods. When the etching is finished, the plate should be very soon coated with wax to protect it. A few hours' exposure to the atmospheric air rusts and destroys the etchings when newly made, although it does not do so afterwards. The oxidation only attacks the lines of the etching, the rest of the plate sustaining no injury, if the air is tolerably dry".
By the process of M. Niepce (Heliography) described in the historical sections, some etchings on metal plates were obtained. Recently his nephew, M. Niepce de St. Victor, has been returning to this process with some success—being assisted in the etching process by M. Lemaitre. These gentlemen have thus described their process:—
"The steel to be operated on having been freed from grease by whitening, M. Lemaitre pours upon the polished surface water acidulated with hydrochloric acid, in the proportions of 1 part of acid to 20 parts of water : this is what he does in aquafortis engraving before applying the varnish; by this means the varnish is made to adhere perfectly to the metal. The plate must then be well washed with pure water and dried. Next, by means of a roll of cloth covered with leather, he spreads upon the polished surface the bitumen of Juchæa dissolved in essential oil of lavender, submits the varnish thus applied to a moderate heat, and when dry, preserves the plate from the action of light or damp.
" On a plate thus prepared, I apply the face of a direct (or positive) photographic proof upon albuminized glass or waxed paper, and expose it to the light during a length of time, varying according to the nature of the proof to be reproduced, and the intensity of the light ; in any case the operation is never very long ; for a proof may be obtained in a quarter of an hour in the sun, and in one hour in diffuse daylight. Indeed it is necessary to avoid prolonging the exposure too much, for in such case the image becomes visible before the operation of the solvent, which is a sign that the proof is a failure, because the solvent will no longer produce any effect.
"I use, as solvent, 3 parts of rectified oil of naphtha and 1 part of benzine (prepared by Colas). These proportions have usually given me good results ; but they may be varied, in proportion to the thickness of the layer of varnish and the time of exposure to light, for the more benzine there is the more solvent the action. The essential oils produce the same effect as benzine, that is to say, they remove the portions of varnish which have been preserved from the action of light. I have discovered that ether acts in the opposite way.
" In order quickly to arrest the action and remove the solvent, I throw water upon the plate so as to form a sheet, and I thus remove the whole of the solvent. I then dry the drops of water which remain upon the plate, and the heliographic operations are terminated.
" It now remains to speak of the operations of the engraver. M. Lemaitre has undertaken to describe them.
" Composition of the mordant or biting liquid :
Nitric acid............36°.........1 part (by volume).
Distilled water........36°........8 „ „
" The action of the nitric acid, diluted with water and alcoholized in these proportions, commences immediately the mordant is poured upon the steel plate, prepared in the manner just described, while the same quantities of nitric acid and water, without alcohol, have the inconvenience of remaining inert for at least two minutes after contact ; I leave the strong mordant only a very short time upon the plate, I remove it, wash and well dry the varnish and the engraving, so as to be able to continue and bite more deeply into the metal without injuring the heliographic layer. For that purpose I use resin reduced to very fine powder ; placed at the bottom of a box prepared for this purpose, I agitate it by means of a pair of bellows, so as to form a sort of cloud of dust which is allowed to settle upon the plate, as is the practice in aqua-tint engraving. The plate is then heated ; the resin forms a network all over the engraving, and it consolidates the varnish, which is then capable of resisting for a long time the corrosive action of the mordant (nitric acid diluted with water, without addition of alcohol). It forms on the blacks a fine grain, which receives the printing ink, and enables us to obtain good and numerous impressions after the varnish and the resin have been removed by the aid of heated greasy substances and essential oils.
"The result of all these operations is, that, without the help of the burin, we may reproduce and engrave on steel all photographic impressions on glass or paper, and without employing the camera obscura".
Processes analogous to these have been employed to obtain impressions upon lithographic stones, which, having been treated in the usual manner, are rendered capable of producing any number of impressions.