A Manual Of Photography

FERROCYANIDE OF POTASSIUM. copy paper, but such as has a good body. I have tried the same paper with the original preparation, and find it to answer ex- ceedingly well; it does not require in this case, either, an acid solution. The same precautions and hints apply to the amended as to the original process ; such as, when it blackens in the dark, there is too much caustic used ; when it remains yellow, or that it is studded with yellow spots, too much iodine ; when marked with black spots, too much iron. It is necessary to mention these, on account of the varying strength of the materials em- ployed.” Section IV.—Ferrocyanide of Potassium. At the meeting of the British Association at Plymouth in 1841 I first directed attention to the use of the ferroprussiate of potash in combination with the iodide of silver. The process resulting from this being very important in many points, the abstract of the paper then read, as given in the Transactions ot the Sections, is reprinted., The author having been engaged in experiments on those varieties of photographic drawings which are formed by the action of the hydriodic salts on the darkened chloride of silver, and with a view to the removal of the iodide formed by the process from the paper, was led to observe some peculiar changes produced by the combined influences of sunshine and the ferrocyanate of potash. It was found that the ordinary photographic paper, if allowed to darken in sunshine, and then slightly acted on by any hydriodic salt, and, when dry, washed with a solution of the ferrocyanate of potash, became extremely sensitive to light, changing from a light brown to a full black by a moment s ex- posure to sunshine. Following out this result, it was discovered that perfectly pure iodide of silver was acted on with even greater rapidity, and thus it became easy to form an exquisitely sensitive photographic paper. The method recommended is the following :— Highly glazed letter paper is washed over with a solution ot one drachm of nitrate of silver to an ounce of distilled water; it is onieklv dried, and a second time washed with the same so- lution It is then, when dry, placed for a minute in a solution of two drachms of the bydriodate of potash in six ounces of water placed on a smooth board, gently washed by allowing some water to flow over it, and dried in the dark at common temperatures. Papers thus prepared may be kept for any length of time and are at any time rendered sensitive by simply wash-