A Manual Of Photography

76 HISTORY OF PHOTOGRAPHY. give it perfect whiteness. If an engraving is to he copied we proceed in the usual manner ; and we may either bring out the picture by placing the paper in a solution of carbonate of soda or potash, by which all the shadows are represented by the chromate of copper, or by washing the paper with nitrate of silver. It may sometimes happen that, owing to deficient light, the photograph is darkened all over when the silver is applied: tins colour, by keeping, is gradually removed, and the picture comes out clear and sharp. If the chromate of copper is dissolved in ammonia, a beautiful green solution results, and if applied to paper acts similarly to those just described. The chromatype pictures, under certain conditions, afford a beautiful example of the changes which take place, slowly, in the dark, from the combined operations of the materials employed. If we take a chromatype picture after it has been developed by the agency of either nitrate of silver, or of mercury, and place it aside in the dark, it will be found, after a few weeks, to have darkened considerably botli in the lights and shadows. This darkening slowly increases, until eventually the picture is obli- terated beneath a film of metallic silver or mercury ; but, while the picture has been fading out on one side, it has been develop- ing itself on the other, and a very pleasing image is seen on the back. After some considerable time the metal on the front gives way again, the paper slowly whitens, and eventually the image is presented on both sides of the paper of equal intensity, in a good neutral tint upon a grey ground. These results, it will be remembered, are of a very similar character to those already described as peculiar to the amphitype process of Sir John Herschel. Section II.—The Ferrotype. Tilis process, which is of remarkable sensibility, was discovered by the author, and published in the Athenaeum, under the name of the Energiatype ; but from a desire to group all those pictures under a general head into which iron salts enter as an element, the present name is preferred. The preparation ol the paper is as follows :—Good letter paper (Whatman’s is the best) is washed over with the following solution, viz. : Five grains of succinic acid (it is important that succinic free from any oil of amber, or adventitious matter, should be obtained) are to be