A Manual Of Photography

298 PRACTICE OF PHOTOGRAPHY. macerated for at least lialf an hour, in a liquid prepared by mixing one part of the already described acid nitrate of mercury, with nine or ten parts of alcohol. A bright lemon yellow precipitate of basic hyponitrate of the protoxide of quicksilver falls, and the clear liquor is preserved for use. The macerated paper is removed from the alcoholic solution, and quickly drawn over the surface of diluted muriatic acid (one part strong acid to seven or ten of water), then quickly washed in water, and slightly and carefully dried at a heat not exceeding 212° of Fahr. The paper is now ready for being bleached by the rays of the sun; and in order to fix the drawing nothing more is required than to steep the paper a few minutes in alcohol, which dissolves the free bichloride of mercury. I must confess, however, that in my hands the process has not been so successful as it is described to have been by the author of it It is perhaps necessary to remark, that we cannot multiply designs from an original hydriodated photograph. The yellow colour of the paper is of itself fatal to transfers, and indepen- dently of this, the wet hydriodic solution would immediately destroy any superposed photograph. We have seen in a former chapter that the white photographic papers are darkened by the blue, indigo, and violet rays. On the dark papers washed with the hydriodic salts in solution, the bleaching is effected most energetically by the violet rays : it proceeds with lessening intensity to the blue, while all the rays below the yellow have a darkening influence on the paper. This effect will be best illustrated by figure 78, in which is shown— somewhat exaggerated for the sake of distinctness—the very remarkable action which takes place ; clearly establishing the fact first noticed by Wollaston, that the two extremities of the spectrum have different powers. The remarkable manner in which the point of greatest inten- sity is shifted from the blue to the violet, when papers have but a very slight difference in their composition or mode of preparation, is an extremely curious point of philosophical inquiry. It will be evident from what has been said, that it is necessary the focus of the violet rays should be always chosen in using the hydriodated papers in the camera.