A Manual Of Photography

1G HISTORY OF PHOTOGRAPHY. been the case. , 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 pro- tected by the varnish remained, of course, unacted on, whilst the other parts were rapidly attacked by the acid. The author 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, com- posed of bitumen from 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 bitu- men 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 al1 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 witli 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 unin- fluenced 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