A Manual Of Photography

ON THE THEORY OF THE DAGUERREOTYPE. 143 perfect pictures were produced, which could not be distinguished from those talten on plates prepared by the ordinary method. So completely does the mixed vapour restore the sensitiveness of prepared plates after exposure to light, that the most beau- tiful impressions were obtained in the camera obscura in two seconds on plates which had previously been four times exposed to the direct light of the sun, and after each such exposure treated with the mixed vapour. “As the plates experimented on, to this stage of the inquiry, had been wholly exposed to the sun's light previous to exposure in the camera obscura, it was thought that possibly some slight effect was produced, which, from being the same on all parts of the plates, escaped observation; and in order to avoid the possi- bility of error from this cause, the impressions of light which it was intended to destroy by bromine were afterwards made in the camera obscura. Prepared plates were impressed with virtual images of different hinds, the camera obscura being pointed first at a house, afterwards to a bust, next to a tree, and finally to a living figure, the plates after each impression, excepting the last, being momentarily exposed to the mixed vapour. In every in- stance, the most perfect impressions of the objects to which the camera obscura was last directed were obtained, and no trace of the previous impressions was left. “Experiments were next instituted for the purpose of ascer- taining if the prepared surface, after the process of mercuriali- zation, could be made to receive another impression bv treatment witli mixed vapour. Impressions were taken with the camera obscura, and after the full development of the picture by vapour of mercury, the plates were exposed to bromine and again placed in the camera obscura, the instrument being directed in different experiments to different objects : on exposure to mercurial vapour, other pictures made their appearance, and although con- fused from superposition on the first pictures, could be clearly traced, and were found perfect in every part. This production of picture upon picture was repeated, until, by the confusion of the superposed images, the effects of further exposure could be no longer distinguished. “In all the experiments hitherto described, the destruction of the impressions by bromine was effected in the dark, the appa- ratus being situated in a room into which only a very feeble day- light was admitted. It remained to be discovered if the mixed vapour had the power of destroying the effect of light while the plate was still exposed to light, or if the va our had the power of suspending or preventing the action of light on a daguerreotype plate. In order to determine this point, the apparatus was