A Manual Of Photography

96 HISTORY 0F PHOTOORAPHY. the sun. It passes also to the same purple hue in the dark, though much more slowly ; so that, as a photographic combina- tion, it is useless. Paper impregnated with acetate of lead, when washed with perfectly neutral chloride of gold, acquires a brownish-yellow hue, and a sensibility to light, which, though not great, is at- tended witli some peculiarities highly worthy of notice. The first impression of the solar rays seems rather to whiten than to darken the paper, by discharging the original colour, and sub- stituting for it a pale greyish tint, which by slow degrees in- creases to a dark slate colour. But if arrested while yet not more than a moderate ash grey, and held in a current of steam, the colour of the part acted on by the sunshine, and that only, darkens immediately to a deep purple. The same effect is pro- duced by immersing it in boiling water. If plunged in cold water, the same change comes on more slowly, and is not com- plete till the paper is dried by heat. A dry heat, however, does not operate this singular change. If a neutral solution of the chloride of gold is mixed with an equal quantity of the solution of bichromate of potash, paper washed with this solution, and exposed to light, speedily changes, first to a deep brown, and ultimately to a bluish black. If an engraving is superposed, we have a negative copy, blue or brown, upon a yellow ground. If this photograph is placed in clean water, and allowed to remain in it for some hours, very singular changes take place. The yellow salt is all dissolved out, and those parts of the paper left beautifully white. All the dark portions become more decided in their character, and according as the solarization has been prolonged or otherwise, or the light has been more or less intense, we have either crimson, blue, brown, or deep black negative photographs. Section X.—Dr. Sciiafhaeutl’s Negative Process. At the tenth meeting of the British Association for the Ad- vancement of Science, two new processes on paper, and one on metal, were brought forward by Dr. Schafhaeutl. These pro- cesses involve some very delicate manipulatory details, which render them tedious, and, in the hands of the inexperienced, un- certain. However, as they sometimes give very perfect results, it would have been improper to have omitted them. Penny's improved patent metallic paper is recommended. This is spread witli a concentrated solution of the nitrate of silver (140 grains to 2| drachms of the fused nitrate, to 6 fluid