A Manual Of Photography

142 SCIENTIFIC INVESTIGATIONS ON PHOTOGRAPHY. exposed part exhibited very little change. , By a careful exami- nation of the plate after it was removed from the mercury box, the white band in the middle appeared to be produced by the feeble light which had passed under the edge of the metal plate which had screened the light from part of the prepared surface; and the very dark, and apparently unaltered appearance of the exposed part, was occasioned by an excess of action, for mercury was found to have condensed on that part in large quantities, and to have produced the dark lead colour which is commonly called solarisation ; but which effect, in the case in question, was so excessive, that the colour of the part on which mercury had condensed differed but very slightly from that on which no light had fallen. It was now evident that the apparent absence of effect in the last experiment was in reality occasioned by an ex- cess of action ; and by repeating that experiment, and making the time of the second exposure to light much shorter than before, the plate assumed, under the action of mercury, an in- tense and beautiful whiteness. “From these experiments, then, it was perfectly clear that the impression produced by the light on a daguerreotype plate is wholly destroyed by the mixed vapour, and that its sensitive- ness to light is restored. “It now remained to discover to what extent tire sensitive- ness is restored by tlie treatment in question. It was not at first expected that the sensitiveness to light was as great after this treatment as after the original preparation of the plate; but experiments afterwards proved that the surface lost none of its sensitiveness by this treatment, nor even by numerous repe- titions of it. A prepared plate was exposed to light; the im- pression was destroyed and sensitiveness restored by the mixed vapour; the plate was a second time exposed to light and a second time to bromine; still its sensitiveness appeared unim- paired, for a fourth or fifth exposure gave, on treatment with mercurial vapour, a vivid impression. In order to determine with the greatest accuracy if the sensitiveness of the prepared surface was at all impaired by these repeated exposures to light, tlie camera obscura was resorted to. A series of plates was pre- pared with the utmost attention to uniformity ; some of these were exposed in the camera obscura, and pictures obtained by the subsequent exposure to vapour of mercury: the time re- quisite for the proper development of the picture was noted; others were first exposed to the direct rays of the sun, and after- wards to tlie mixed vapour, and these were exposed in the camera obscura for the same length of time as those which had not been exposed to light. On treatment with mercurial vapour,