A Manual Of Photography

64 HISTORY OF PHOTOGRAPHY. the same action of the light, of either a positive or a negative picture, according to the subsequent manipulations, that 1 have designated the process thus generally sketched out, by the term "Amphitypea name suggested by Mr. Talbot, to whom I communicated tins singular result ; and to this process or class of processes (which I cannot doubt when pursued will lead to some very beautiful results) I propose to restrict the name in question, though it applies even more appropriately to the fol- lowing exceedingly curious and remarkable one in which silver is concerned. At the last meeting I announced a mode of producing, by means of a solution of silver in conjunction with ferro-tartaric acid, a dormant picture brought out into a for- cible negative impression by the breath, or moist air. The solution then described, and which had at that time been pre- pared some weeks, I may here incidentally remark, has retained its limpidity and photographic properties quite unimpaired during the whole year since elapsed, and is now as sensitive as ever—a property of no small value. Now, when a picture (for example, an impression from an engraving) is taken on paper washed with this solution, it shows no sign of a picture on its back, whether that on its face be developed or not ; but if, while the actinic influence is still fresh upon the face (o e. as soon as it is removed from the light), the back be exposed for a very few seconds to sunshine, and then removed to a gloomy place, a positive picture, the exact complement of the negative one on the other side, though wanting of course in sharpness if the paper be thick, slowly and gradually makes its appearance there, and in half an hour acquires considerable intensity. I ought to mention that the ferro-tartaric acid in question is prepared by precipitating the ferro-tartrate of ammonia by acetate of lead, and decomposing the precipitate by dilute sulphuric acid." Section VI.—The Colouring Matter of Flowers. The results obtained by Sir John Herschel on the colouring juices of flowers are too remarkable to be omitted in a treatise in which it is desirable that every point should be registered up to the date of publication, which connects itself with the pheno- mena of chemical change applied to photography. " In operating on the colours of flowers, I have usually pro- ceeded as follows:—The petals of the fresh flowers, or rather such parts of them as possessed a uniform tint, were crushed to a pulp in a marble mortar, either alone, or with addition of alcohol, and the juice expressed by squeezing the pulp in a