A Manual Of Photography

208 PRACTICE OF PHOTOGRAPHY. its surface a glass rod held in two bent pieces of glass, as in fig. 60; the object of which is to remove the small air-bubbles that form on the surface of the paper, and protect it from the action of the fluid. This process, however well it may answer in preparing paper for copying engravings, will yield paper not sufficiently sensitive for camera purposes ; and it is objection- able on the score of economy, as a larger quantity of the silver solution is required to decompose the common salt titan in the process described. It may not be entirely useless or uninteresting, to state the more striking peculiarities of a few of the washes, on tlie study of which depends the possibility of our ever producing photo- graphs in their natural colours, — a problem of the highest interest. It will be found that nearly every variety of paper exposed to the full action of the solar beams will pass through various sliades of brown, and become at last of a deep olive colour : it must therefore be understood that the process of darkening is in all cases stopped short of this point. Muriate of Soda.—Papers prepared witli the muriate of soda have been more extensively used than any others for positive pictures, owing to the ease with which tills material is alvv ays to be procured; and for most purposes it answers as well as any other, but it does not produce the most sensitive photographic ground. The proportions in which this salt lias been used are exceed- ingly various : in general, the solution has been made too strong; biit several chemists have recommended washes that are as much too weak. For different uses, solutions of various qualities should be employed. It will be found well in practice to keep papers of three orders of sensitiveness prepared; the proportions of salt and silver for eacli being as follows:— Sensitive Paper for the Camera Obscura. Muriate of soda, thirty grains to an ounce of water. Nitrate of silver, one hundred and twenty grains to an ounce of distilled water.. The paper is first soaked in the saline solution, and after being carefully wiped with linen, or pressed between folds of blotting paper and dried, it is to be washed twice witli tlie solution of silver, drying it by a warm fire between eacli washing. Ibis paper is very liable to become brown in tlie dark. Although images may be obtained in the camera on tills paper by abput half an hour’s exposure, they are never very distinct, and may be regarded as rather curious than useful.