A Manual Of Photography

FIXING TUE DAGUERREOTYPE IMAGE. 253 fluid is mixed with three or four volumes of pure water, which is poured into a flat vessel, in sufficient quantity that the plate may he steeped in it horizontally, and just covered with the fluid. When, by the action of the fluid, the tints of the drawing are sufficiently changed, which occurs in less than a minute, the plate is to be withdrawn and put into a ffat vessel containing water : it is afterwards taken out and dried. By this process the polished parts of the metal are tinged grey by the sulphuret, and the amalgamated parts are not attacked,—or, at least, but very little. The tints may be varied by the concentration of the fluid or the duration of the immersion: however, too long an action turns the lights yellow. Photogenic images, treated in this manner, bear rubbing with the finger without losing any of their details. M. Choiselat proposed a plan which has been rarely acted upon, but which is well deserving of all attention. Chloride, and particularly iodide, of silver, dissolved in hypo- sulphite of soda, may be advantageously employed for fixing the images of the daguerreotype. Steeped in these solutions, they are under the electro-chemical influence exerted by the copper on the dissolved silver, and thus became ineffaceable. Instead of the hyposulphite, a mixture of iodide, bromide, &c., of potassa may be employed, holding the silver salt in so- lution. The iodide of silver best adapted for this operation is that which is obtained by treating, with the aid of heat, a plate of this metal by the 'iodide precipitated from alcohol by water, afterwards dissolving the iodide formed and adhering to the plate in the hyposulphite. Dr. Berres, of Vienna, assisted by Mr. F. Kratochwila, has succeeded by another process, bearing some analogy to that of M. Fizeau, in fixing the daguerreotype designs. He takes the photograph produced in the usual manner by the process of Daguerre, holds it for a few minutes over a moderately warmed nitric acid vapour, and then lays it in nitric acid of 13° or 14° Réaumur (61|° or 63|° Fahrenheit), in which a considerable quantity of copper or silver, or both together, has been previously dissolved. Shortly after having been placed therein, a precipi- tate of metal is formed, and can be changed to any degree of intensity. The photographic picture coated with metal is now removed, washed in water, cleaned, and dried ; it is then polished with chalk or magnesia, and a dry soft cloth or leather, after which the coating will become clean, clear, and transparent, so that the picture, with all its properties, can again be seen.