At the meeting of the British Association at Plymouth in 1841, I first directed attention to the use of the ferrocyanide of potassium 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 of 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 ferrocyanide of potassium. It was found that the ordinary photographic paper, if allowed to darken in sunshine, and then slightly acted on by any hydriodic solution, and, when dry, washed with a solution of the ferrocyanide of potassium, became extremely sensitive to light, changing from a light brown to a full black by a moment's exposure 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 of one drachm of nitrate of silver to an ounce of distilled water; it is quickly dried, and a second time washed with the same solution. It is then, when dry, placed for a minute in a solution of two drachms of the iodide of potassium 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 washing them over with a solution formed of one drachm of the ferrocyanide of potassium to an ounce of water.

These papers, washed with the ferrocyanide and dried in the dark, are, in this dry state, absolutely insensible, but they may at any moment be rendered sensitive by merely washing them with a little cold clean water.

Papers thus prepared are rendered quite insensible by being washed over with the above hydriodic solution. They are, however, best secured against the action of time by a solution of ammonia.