" In one of these trials the light acted in such a way that the varnish was removed in proportion to the intensity with which the light had acted, and the picture exhibited a more marked gradation of tone ; so that, viewed by transmitted light, the landscape produced, to a certain extent, the well-known effects of the diorama.
" In the second trial, on the contrary, the action of the luminous fluid having been more intense, the parts acted upon by the strongest lights, not having been attacked by the solvent, remained transparent; the difference of tone resulted from the relative thickness of the coatings of varnish.
" If this landscape is viewed by reflection in a mirror, on the varnished side, and at a certain angle, the effect is remarkably striking; while, seen by transmitted light, it is confused and shapeless : but, what is equally surprising, in this position the mimic tracery seems to affect the local colour of the objects".
A statement that M. Niepce was enabled to engrave by light, went the round of the press; but this does not appear to have been the ease. All that the author of heliography effected, was the etching of the plate, after it had undergone its various processes, and the drawing was completed by the action of nitric acid in the usual manner : the parts of the copperplate protected by the varnish remained, of course, unacted on, whilst the other parts were rapidly attacked by the acid. Niepce remarks that his process cannot be used during the winter season, as the cold and moisture render the varnish brittle, and detach it from the glass or metal.
M. Niepce afterwards used a more unctuous varnish composed of bitumen of Judea, dissolved in animal oil of Dippel. This composition is of much greater tenacity and higher colour than the former, and, after being applied, it can immediately be submitted to the action of light, which appears to render it solid more quickly, from the greater volatility of the animal oil. M. Daguerre remarks, that this very property diminishes still further the resources of the process as respects the lights of the drawings thus obtained. These processes of M. Niepce were much improved by M. Daguerre, who makes the following remarks on the subject:—
" The substance which should be used in preference to bitumen is the residuum obtained by evaporating the essential oil of lavender, which is to be dissolved in alcohol, and applied in an extremely thin wash. Although all bituminous and resinous substances are, without any exception, endowed with the same property—that of being affected by light—the preference ought to be given to those which are the most unctuous, because they give greater firmness to the drawings. Several essential oils lose this character when they are exposed to too strong a heat.
" It is not, however, from the ease with which it is decomposed, that we are to prefer the essential oil of lavender. There are, for instance, the resins, which, being dissolved in alcohol, and spread upon glass or metal, leave, by the evaporation of the spirit, a very white and infinitely sensitive coating. But this greater sensibility to light, caused by a quicker oxidation, renders also the images obtained much more liable to injury from the agent by which they were created. They grow faint, and disappear altogether, when exposed but for a few months to the sun. The residuum of the essential oil of lavender is more effectually fixed, but even this is not altogether uninfluenced by the eroding effects of a direct exposure to the sun's light.
" The essence is evaporated in a shallow dish by heat, till the resinous residuum acquires such a consistency, that when cold. it rings on being struck with the point of a knife, and flies off in pieces when separated from the dish. A small quantity of this material is afterwards to be dissolved in alcohol or ether; the solution formed should be transparent, and of a lemon-yellow colour. The clearer the solution, the more delicate will he the coating on the plate : it must not, however, be too thin, because it would not thicken or spread out into a white coat; indispensable requisites for obtaining good effects in photographic designs. The use of the alcohol or ether is to facilitate the application of the resin under a very attenuated form, the spirit being entirely evaporated before the light effects its delineations on the tablet. In order to obtain greater vigour, the metal ought to have an exquisite polish. There is more charm about sketches taken on glass plates, and, above all, much greater delicacy.
"Before commencing operations, the experimenter must carefully clean his glass or metal plate. For this purpose, emery, reduced to an impalpable powder, mixed with alcohol, may be used ; applying it by means of cotton-wool: but this part of the process must always be concluded by dry-polishing, that no trace of moisture may remain on the tablet. The plate of metal or glass being thus prepared, in order to supply the wash or coating, it is held in one hand, and with the other, the solution is to be poured over it from a flask or bottle having a wide mouth, so that it may flow rapidly, and cover the whole surface. It is at first necessary to hold the plate a little inclined; but as soon as the solution is poured on, and has ceased to flow freely, it is raised perpendicularly. The finger is then passed behind and below the plate, in order to draw off a portion of the liquid, which, tending always to ascend, would double the thickness of the covering: the finger must be wiped each time, and be passed very rapidly along the whole length of the plate from below, and on the side opposite the coating. When the liquid has ceased to run, the plate is dried in the dark. The coating being well dried, it is to be placed in the camera obscura. The time required to procure a photographic copy of a landscape is from seven to eight hours; but single monuments strongly illuminated by the sun, or very bright in themselves, are copied in about three hours.
" When operating on glass it is necessary, in order to increase the light, to place the plate upon a piece of paper, with great care that the connection is perfect over every part, as, otherwise, confusion is produced in the design by imperfect reflection.
" It frequently happens that when the plate is removed from the camera, there is no trace of any image upon its surface: it is therefore necessary to use another process to bring out the hidden design.
" To do this, provide a tin vessel, larger than the tablet, having all round a ledge or border 50 millimeters (2 English inches) in depth. Let this be three-quarters fall of the oil of petroleum; fix your tablet by the back to a piece of wood which completely covers the vessel, and place it so that the tablet, face downwards, is over but not touching the oil. The vapour of the petroleum penetrates the coating of the plate in those parts on which the light has acted feebly; that is, in the portions which correspond to the shadows, imparting to them a transparency as if nothing were there. On the contrary the points of the resinous coating, on which light has acted, having been rendered impervious to the vapour, remain unchanged.
"The design must be examined from time to time, and withdrawn as soon as a vigorous effect is obtained, By urging the action too far, even the strongest lights will be attacked by the vapour, and disappear, to the destruction of the piece. The picture, when finished, is to be protected from the dust by being kept covered with a glass, which also protects the silver plate from tarnishing".
It may perhaps appear to some that I have needlessly given the particulars of a process, now superseded by others, possessing the most infinite sensibility; producing in a few minutes a better effect than was obtained by the heliographic process in several hours. There are, however, so many curious facts connected with the action of light on these resins, that no treatise on photography could be considered complete without some description of them; and this process is now revived with a view to the production of etchings directly from nature.
M. Daguerre remarks, that numerous experiments tried by him with these resinous preparations of M. Niepce, prove that light cannot fall upon a body without leaving traces of decomposition; and they also demonstrate that these bodies possess the power of renewing in darkness, what has been lost by luminous action, provided total decomposition has not been effected. This heliographic process must be regarded as the earliest successful attempt at fixing on solid tablets the images of the camera ob-scura, and at developing a dormant image.