A Manual Of Photography

138 SCIENTIFIC INVESTIGATIONS ON PHOTOGRAPHY. on the plate being in exact proportion to the chemical agency of the coloured ray by which it was decomposed. On photographic papers, the decomposed argentine salt exists, in al1 probability, in a state of oxide, mixed with revived silver ; but on the silver tablet the iodine is changed over all the parts on which the light acts, and pure silver in a state of extreme division results. The depth to which the decomposition has been effected being in exact relation to the intensity and colour of the light radiated from the object which we desire to copy, the mercurial vapour unites with different proportions of silver, and thus are formed the lights and middle tints of the picture. The shadows are produced by the unchanged silver from which the iodine is removed by the hyposulphite of soda. Daguerre himself laid much stress upon the necessity of ex- posing the plate to receive the vapour of mercury at an angle of 45°. This, perhaps, is the most convenient position, as it enables the operator to view the plate distinctly, and watch the develop- ment of the design : but beyond this, I am satisfied there exists no real necessity for the angular position. Both horizontally and vertically, I have often produced equally effective daguerreo- types. Looking at a daguerreotype picture in such a position that the light is incident and reflected at a large angle, the drawing appears of the negative character ; the silver in such a position appearing white, and the amalgam of mercury and silver a pale grey. View the plate in any position which admits of but a small angle of reflection, and we then see the design in all its exquisite beauty, correct in the arrangement of its lights and shades,—the silver appearing black, while the amalgam, by contrast in part, and partly in reality, appears nearly white. A very ingenious idea has been promulgated, that the light crystallizes the iodide of silver, and that the mercury adheres to one of the facets of each minute crystal. If this was the case, the picture could be seen distinctly in one position only, whereas in many different positions it is equally clear. There does not appear to be any more difficulty in explaining why the mercu- rial amalgam should vary in its tint with change of position, than in explaining why a common mirror, or a polished metal plate, should appear white when viewed at one angle, and black in another. The cause leading to the uniform deposition of the mercurial vapour is far more difficult of solution. It does not appear to me that any one of the hypotheses put forth, satisfies all the con- ditions of this peculiar phenomenon. Few papers have been published which so completely investi- gate the phenomena of the chemical change in the daguerreotype,