A Manual Of Photography

74 HISTORY OF PHOTOGRAPHY. Ponton’s manner, and upon which there exists a faint copy of a drawing, in a solution of iodine in alcohol, to wash this paper in alcohol, and then dry it: then the parts which were white become blue, and those which were yellow remain more or less clear. M. E. Becquerel lias pursued his investigations into the action of the chromic acid on organic compounds, and has shown that the mode of sizing the papers influences their colouration by light, and that with unsized paper colouration is effected only after a long time. Perceiving that the principal reaction resulted from the chromic acid contained in the bichromate of potash, on the starch in the size of the paper, it occurred to M. E. Becquerel, that, as starch has the property of forming witli iodine a combi- nation of a very fine blue colour, it should produce deep shades of that tint, whilst the lights still remained an orange- yellow. His method of proceeding is to spread a size of starch very uniformly over the surface of the paper. It is then steeped in a weak alcoholic solution of iodine, and afterwards washed in a great quantity of water. By this immersion it should take a very fine blue tint. If this is uniform, the paper is considered fit for the experiment: in the contrary case it is sized again. It is then steeped in a concentrated solution of bichromate of potash, and pressed between folds of blotting paper, and dried near the fire. To be effective, it should be very dry. It is now fit for use. When the copy is effected, which re- quires in sunshine about five minutes, the photograph is washed and dried. When dry, it is steeped in a weak alcoholic solution of iodine, and afterwards, when it has remained in it some time, it is washed in water, and carefully dried with blotting paper, but not at the fire, for at a little below 100° Bahr, the combination of iodine and starch discolours. If it be considered that the drawing is not sufficiently distinct, this immersion may be repeated several times ; for by this means may be obtained the intensity of tone that is desired, which intensity can be changed at will by employing a more concentrated solution of iodine. When the paper is damp, the shades are of a very fine blue, but when it is dry the colour becomes deep violet. If while the drawing is still wet it be covered with a layer of gum arable, the colour of the drawing is greatly preserved, and more beautiful when it is dry. When a paper is thus prepared it loses at first a little of its tone, but it afterwards preserves its violet tint. The Chromatype Process.—This process, devised by the author, is a pleasing one in its results: it is exceedingly simple