A Manual Of Photography

66 HISTORY OF PHOTOGRAPHY. green; and a beautiful rose-coloured tulip, a dirty bluish green; but perhaps the most striking case of this kind is »that of a common sort of red poppy {Papaver Rheum), whose expressed juice imparts to paper a rich and most beautiful blue colour, whose elegant properties as a photographic material will be further alluded to hereafter.1 “ Tins change of colour is probably owing to different causes in different flowers. In some it undoubtedly arises from the escape of carbonic acid, but this, as a general cause for the change from red to blue, has, I am aware, been controverted. In some (as is the case with the yellow ranunculi) it seems to arise from a chemical alteration depending on absorption of oxygen; and in others, especially where the expressed juice coagulates on stand- ing, to a loss of vitality or disorganization of the molecules. The fresh petal of a single flower, merely crushed by rubbing on dry paper, and instantly dried, leaves a stain much more nearly approximating to the original hue. This, for example, is the only way in which the fine blue colour of the common field veronica can be imparted to paper. Its expressed juice, however quickly prepared, when laid on with a brush, affords only a dirty neutral grey, and so of many others. But in this way no even tint can be had, which is a first requisite to the experiments now in ques- tion, as well as to their application to photography. “ To secure this desirable evenness of tint, the following mani- pulation will generally be found successful:—The paper should be moistened at the back by sponging and blotting off. It should then be pinned on a board, the moist side downwards, so that two of its edges (suppose the right-hand and lower ones) shall project a little beyond those of the board. The board being then inclined twenty or thirty degrees to the horizon, the alco- holic tincture (mixed with a very little water, if the petals them- selves be not very juicy) is to be applied with a brush in strokes from left to right, taking care not to go over the edges which rest on the board, but to pass clearly over those which project, and observing also to carry the tint from below upwards by quick sweeping strokes, leaving no dry spaces between them, but keeping up a continuity of wet surface. When al1 is wet, cross them by another set of strokes from above downwards, so managing the brush as to leave no floating liquid on the paper. It must then be dried as quickly as possible over a stove, or in a current of warm air ; avoiding, however, such heat as may 1 A semicultivated variety was used, having dark purple spots at the bases of the petals. The common red poppy of the chalk (Papaver hybridum) gives a purple colour much less sensitive and beautiful.