38 C.-rOSITIVE PHOTOGRAPHS BY MEANS OF IIYDRIODIC SALTS.

instantly exposed to sunshine, and, if practicable, carried into the openair, as the more speedily evaporation proceeds, the less does the silverpenetrate the paper, and the more delicate it is. The first surface isvery irregular, being as before described, and represented in figure 2.As soon as the surface appears dry, the silver solution must be againapplied as before, and the exposure repeated. It must now be exposeduntil a fine chocolate-brown colour is produced equally on all parts ofthe surface, and then, until required for use, be carefully preservedfrom the further influence of light. If the paper is to be kept long,the darkening must not be allowed to proceed so far as when it is tobe speedily made use of.

In darkening these papers, tlio greatest possible attention must bopaid to the quantity of light to which they are submitted, every thingdepending on the rapidity of the blackening process. The morning sunshould be chosen, for the reasons before stated. A perfectly cloudlesssky is of great advantage. The injurious consequence of a cloud ob-scuring the sun during the last darkening process, is the formation of asurface which has the appearance of being washed with a dirty brush.This is with difficulty romovod by the hydriodates, and the resultingpictures want that clearness which constitute their beauty. Papersdarkened 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 silversolution from flowing over the edges of the paper, as thereby an extraquantity of darkened silver is formed on both sides, which requires along-continued action of the hydriodates and light to bleach.

The kind of paper on which the silver is spread, is an object of muchimportance. A paper known to stationers as satin post, double-glazed,bearing the mark of J. Whatman , Turkey Mill, is decidedly superior toevery other kind I have tried. The dark specks which abound in somesorts of paper must be avoided, and the spots made by flies very care-fully guarded against. These are of small consequence during thedarkening process, but when the hydriodic wash is applied, they formcentres of chemical action, and the bleaching process goes on aroundthem, independently of light, deforming the drawing with small rings,which aro continually extending their diameters.

The saline washes may be considerably varied, and combined to anindefinite extent, with a continued change of effect, which is singularlyinteresting. In their application we should be guided, as in the nega-tive process, by their combining proportions. The following list of thesalts which will give the best effects, selected from upwards of sevenhundred combinations, will show the variety of colours produced. Theyare placed in the order of the sensitiveness they appear to maintain,when used as nearly as possible under the samo circumstances.

Colour of Picture.

Muriate of Ammonia, . Red , changing to black in the sunshine.

Chloride of Sodium, . Ditto. ditto.

Muriate of Stkoxtia . A fine brown.