A Manual Of Photography

CHAPTEE III. THE THEORY OF THE DAGUERREOTYPE. Numerous speculations having been ventured as to the pecu- liar chemical changes which light produces on the iodidated silver tablets, I shall make no apology for introducing a few remarks on this very interesting subject. Numerous experiments on plated copper, pure silver plates, and on silvered glass and paper, have convinced me that the first operations of polishing with nitric acid, &c., are essential to the production of the most sensitive surface. All who will take the trouble to examine the subject will soon be convinced that the acid softens the silver, bringing it to a state in which it is ex- tremely susceptible of being either oxidized or iodized, according as the circumstance may occur of its exposure to the atmosphere or to iodine. The process, adopted, I believe, first in America, of producing a deposit of chemically pure silver on the plated metal, by means of the voltaic battery, which certainly gives rise to some peculiar conditions, appears to prove that the soft surface of silver is of advantage. The sensitive surface is a combination of iodine, or of iodine and bromine, with the silver. When exposed to radiant in- fluences in the camera, a molecular change is effected, and there is mucli doubt if any iodine or bromine is removed from the surface. Some have thought that the superficial film being decomposed, the iodine and bromine attack a lower surface of the plate ; but experiments are still wanting. I have discovered that all the rays of the prismatic spectrum act on the daguerreotype plate, except the yellow, and a circle of light of a peculiar and mysterious character, which surrounds the visible spectrum. The light acting on a prepared tablet, appears to decompose the film of ioduret of silver to different depths, according to the order of refrangibility of the rays : the violet ray and extra-spectral rays effecting the deepest decom- position," whilst the red acts to a depth inappreciably slight. Thus it is that the spectrum impressed on a daguerreotype plate reflects natural tints of the same kind as Sir Isaac Newton's thin films; the thickness of each film of reduced silver