The advance of photography : its history and modern applications (1911)

28 THE ADVANCE OF PHOTOGRAPHY The negative must, therefore, be more strongly defined, and this is produced by repeating the developing process. A mixture of solutions of green vitriol and of nitrate of silver is poured upon the picture, a silver precipitate is again formed on it, which adheres only to the dark parts of the negative, giving them a greater densit}^. If the plate is not perfectly clean, silver is precipitated, in the processes of developing and intensifying, upon the dirt stains and produces spots. After the further development of the picture, or the so-called intensifying process, has been completed, it is only necessary to remove the iodide of silver, which diminishes the transparency of the clear parts of the plate. For this purpose a solution of hyposulphite of soda is poured on the plate. This salt has the property of dissolving salts of silver, which are insoluble in water, so that the iodide of silver vanishes under the influence of this solution. This is the fixing process. Lastly, the plate is washed and dried. It must be borne in mind that all these different operations are performed on a film liable to be injured by the slightest touch. Even when dried the picture is very liable to injury ; and therefore it is necessary, in order to protect it, to cover it with a varnish — that is, with a solution of a resin, such as shellac or sandarach, in spirits of wine. The fragile glass negative is therewith brought to completion. The chief requisite for the success of these operations is routine ; accuracy is only attained by practising each part of the process. Faults that are made in any single operation are, as a general rule, irremediable ; and therefore it is absolutely essential to avoid them. It was next necessary to obtain a positive from the negative. The old Talbot method Avas employed for that purpose. The positive was neither a correct nor an agreeable picture of the object, but often showed considerable