A Manual Of Photography

18 HISTORY OF PHOTOGRAPHY. is therefore necessary to use another process to bring out the hidden design. Io do this, provide a tin vessel, larger than the tablet, having all round a ledge or border 50 millimeters (2 English inches) in depth. Let this be three quarters full of the oil of petroleum: fix your tablet by the back to a piece of wood which completely covers the vessel, and place it so that the tablet, face downwards is over but not touching the oil. The vapour of the petroleum penetrates the coating of the plate in those parts on which the light has acted feebly ; that is, in the portions which correspond to the shadows, imparting to them a transparency, as if nothin were there. On the contrary, the points of the resinous coating on which light has acted, having been rendered impervious to the vapour, remain unchanged. “ The design must be examined from time to time, and with- drawn as soon as a vigorous effect is obtained. By urging the action too far, even the strongest lights will be attacked by the vapour, and disappear, to the destruction of the piece. The picture, when finished, is to be protected from the dust by bein^ kept covered witli a glass, which also protects the silver plate from tarnishing."1 It may perhaps appear to some that I have needlessly given the particulars of a process, now entirely superseded by others possessing the most infinite sensibility ; producing in a few minutes a better effect than was obtained by the Heliographie process in several hours. There are, however, so many curious facts connected with the action of light on these resins that no treatise on photography could be considered complete'without some description of them. .21. Daguerre remarks, that numerous experiments tried by bun with these resinous preparations of M. Niepce, prove that light cannot fall upon a body without leaving traces of decom- position ; and they also demonstrate that these bodies possess the power of renewing in darkness, what has been lost by lumi- nous action, provided total decomposition has not been effected Ihis hellographic process must be regarded as the earliest suc- cessful attempt at fixing on solid tablets the images of the camera-obscura, and at developing a dormant image. As M. Niepce appears to have allowed the investigation after this period to fall into the hands of Daguerre, further remarks are reserved for the chapter devoted to the history of the daguer- reotype. b