A Manual Of Photography

CHAPTER IV. ON FIXING THE PHOTOGRAPHIC PICTURES. The power of destroying the susceptibility of a photographic agent to the further action of light, when the picture is com- pleted by its influence, is absolutely necessary for the perfection of the art. Various plans have been suggested for accomplishing this, which have been attended with very different results ; few, if any, of the materials used producing the required effect, and, at the same time, leaving the picture unimpaired. The hyposulphite of soda is decidedly superior to every other fixing material ; but it will be interesting to name a few other preparations, which may be used with advantage in some instances. The pictures formed on papers prepared with the nitrate of silver only, may be rendered permanent by washing them in very pure water. The water must be quite free from any muriates, as these salts attack the picture with considerable energy, and soon destroy it, by converting the darkened silver into a chloride, which changes upon exposure. The great point to be aimed at in fixing any of the sun- pictures is the removal of all that portion of the preparation, whatever it may be, which has not undergone change, without disturbing those parts which leave been altered in the slightest degree by the chemical radiations. When a picture has been obtained upon paper prepared with the nitrate of silver, or the ammonio-nitrate of silver, the best mode of proceeding is to wash it first with warm rain water, and then with a diluted solution of ammonia: if the ammonia is too strong, it dissolves the oxide of silver, which in these processes is formed in the fainter parts of the picture, and thus obliterates the more delicate portions. Herschel remarks—“ If the paper be pre- pared with the simple nitrate, the water must be distilled, since the smallest quantity of any muriatic salt present attacks the picture impressed on such paper with singular energy, and speedily obliterates it, unless very dark- A solution containing only a thousandth part of its weight of common salt suffices to effect this in a few minutes in a picture of considerable strength." Photographs on the muriated papers are not, however, so