A Manual Of Photography

ON THE PRODUCTION OF POSITIVE PHOTOGRAPHS. 289 published in the London and Edinburgh Philosophical Magazine for September and October 1840, to which will be added the results of my experiments since that time. The variable texture of the finest kinds of paper occasioning irregularities of imbibition, is a constant source of annoyance, deforming the drawings with dark patches, which are very difficult to remove: consequently my first endeavours were directed to the formation of a surface on which the photographic preparations might be spread with perfect uniformity. A variety of sizes were used with very uncertain results. Nearly all the animal glutens appear to possess a colorific property, which may render them available in many of the negative processes; but they al1 seem to protect the darkened silver from the action of the hydriodic solutions. The gums are acted on by the nitrate of silver, and browned, independent of light, which browning considerably mars the effect of the finished picture. It is a singular fact, that the tragacanth and acacia gums render the gums much less permanent. I therefore found it necessary for general practice to abandon the use of al1 sizes, except such as enter into the composition of the paper in the manufacture. It occurred to me that it might be possible to saturate the paper with a metallic solution, which should be of itself entirely uninfluenced by light, on which the silver coating might be spread without suffering any material chemical change. The results being curious, and illustrative of some of the peculiarities of the hydriodic salts, it will be interesting to study a few of them. Sulphate and Muriate of Iron.—These salts, when used in small proportions, appeared to overcome many of the first difficulties, but all the drawings on papers thus prepared faded out in the dark. If, after these photographs have faded entirely out, they are soaked for a short time in a solution of the ferro- cyanate of potash, and then are exposed to the light, the picture is revived, but with reversed lights and shadows. Acetate and Nitrate of Lead —These salts have been much used bv Sir John Herschel, both in the negative and positive processes, and, it appears, with considerable success. I found a tolerably good result when I used a saturated solution ; but papers thus prepared required a stronger light than other kinds. When I used weaker solutions, the drawings were covered with black patches. On these a little further explanation is required. When the strong solution has been used, the hydriodate acid which has not been expended in forming the iodide of silver— which forms the lights of the picture—goes to form the iodide iodide is sole )iling water, and is easily