A Manual Of Photography

ON THE THEORY OF THE DAGUERREOTYPE. 139 as that of Mr. George Shaw, who pursued his experiments in association with Dr. Percy. As giving a large amount of valuable information, I transfer it from the Philosophical Magazine. “ It is well known that the impression produced by light on a plate of silver rendered sensitive by M. Daguerre's process, is wholly destroyed by a momentary exposure of the plate to the vapour of either iodine or bromine. Although, this fact has long been known, the nature of the action by whicli so extra- ordinary an effect is produced has not yet been satisfactorily explained. In the hope of elucidating this subject, a series of experiments was instituted, the results of which are recorded in the following remarks. "A silver plate prepared by exposure to iodine or its com- pounds with bromine, may be exposed to the vapour of mercury without being in any way affected by the exposure. If, however, the prepared plate be previously exposed to light, or made to receive the luminous image formed in the camera obscura, the mercurial vapour attacks it ; forming, in the former case, a white film, and in the latter, a picture corresponding to the luminous image which had been allowed to fall on it. “ If a prepared plate, after receiving a vertical impression by light, be exposed to the vapour of iodine or bromine, it is found that the vapour of mercury no longer attacks it ; or, in other words, the impression produced by light is destroyed. “The first experiments made for the purpose of arriving at the cause of this phenomenon had reference to the relation between the time of the exposure to light and the time of ex- posure to the vapour of iodine or bromine necessary to destroy the effect produced by light. Prepared plates were exposed in the camera obscura for a length of time, whicli previous experi- ment had determined to be sufficient for a full development of the picture ; some of those plates were exposed during two seconds to an atmosphere feebly charged with the vapour of bromine, while others were carefully preserved from contact with the vapours of iodine or bromine. The atmosphere of bromine employed was produced by adding thirty drops of a saturated solution of bromine in water to an ounce of water : the solution was poured into a glass vessel, and the plate was exposed to the vapour in the vessel during the time specified. The plates were then introduced into the mercury box, and by volatilizing the metal, pictures were developed on all those which had not been exposed to the vapour of bromine, while those whicli had been exposed to it exhibited no trace of a picture under the action of mercury.