A Manual Of Photography

POSITIVE PHOTOGRAPHS BY ONE PROCESS. 89 Papers darkened by the diffused light of a cloudy day are scarcely, if at all, acted on by these salts. Great care must be taken to prevent the silver solution from flowing over the edges of the paper, as thereby an extra quantity of darkened silver is formed on both sides, which requires a long-continued action of the hydriodates and light to bleach. The kind of paper on which the silver is spread is an object of much importance. A paper known to stationers as satin post, double-glazed, bearing the mark of J. Whatman, Turkey Mill, is decidedly superior to every other kind I have tried. The dark specks which abound in some sorts of paper must be avoided, and the spots made by flies very carefully guarded against. These are of small consequence during the darkening process, but when the hydriodic wash is applied, they form centres of chemical action, and the bleaching process goes on around them, independently of light, deforming the drawing with small rings, which are continually extending their diameters., The saline washes may be considerably varied, and combined to an indefinite extent, with a continued change of effect, which is singularly interesting. In their application we should be guided, as in the negative process, by their combining propor- tions. The following list of the salts which will give the best effects, selected from upwards of seven hundred combinations, will show the variety of colours produced. They are placed in the order of the sensitiveness they appear to maintain, when used as nearly as possible under the same circumstances. Colour of Picture. Muriate or Ammonia. . Red, changing to black in the sunshine. Chloride or Sodium... Ditto. ditto. Muriate or Strontia. . A fine brown. Muriate of Baryta .. .A rich brown, inclining to purple. Sol. Chloride of Lime Very red. Sol. Chloride of Soda A brick red. Iodide of Potassium. ..Yellowish brown. _„Variable, sometimes yellowish, often a Chlorate or Potash^ steel blue^ Phosphate of Soda.... Mouse colour. Tartrate of Soda.....Dark brown. Urate of Soda........Yellowish brown. IMuriate of Iron.....Deep brown, which blackens. Bromide of Sodium.... Red brown, of a peculiarly rich tint. The change mentioned in the colour of the finished picture