DAGUERREOTYPE PROCESS. This is one :of• the tiarliest methods knovin for fixing the. image, afforded by 'the camera, and thus producing permanent pictures, or "photographs" As practised by Daguerre (q.v.), the method consisted in exposing a silver plate to the action of iodine vapor until a suf ficient coating of iodide of silver was produced upon it, the image in the camera being then ex posed upon it for a time varying from three minutes to an hour or more, according to the nature of the subject and the intensity of the light. The plate was then submitted to the action of the vapor of mercury, 'which con densed most upon the parts where the light had acted most, and in this way greatly increased the distinctness of the image. The after development by mercury vapor in this way, was immersed in a solution of hyposulphite of soda, which dissolved those parts of the iodide that had not been affected by light, and thus ren dered the picture permanent. Daguerre made the details of his process public in 1839, and for this he was awarded a pension by the French government. Important improvements in the method were soon made. In December 1839, before the first French photograph by the iodine process had been received in the United States, Dr. Paul Beck Goddard of Philadelphia discovered that iodine could be advantageously replaced by bromine, an element discovered by Balard 13 years before. By the use of bromine, the sensitiveness of the plate was increased so greatly that Dr. Goddard obtained some prac tically instantaneous views. Early in 1840 Rob ert Cornelius, of Philadelphia, fitted up a room exclusively for portraiture, and this was the first photographic studio in the world. The expensive silver plates were afterward replaced by plates of copper that had been heavily elec.; troplated with silver, and a method of toning the pictures by the use of chloride of gold was also devised. Daguerre was associated, in his experimental work, with Nicephore Niepce, who had previously discovered the bitumen process (q.v.) of taking photographs; and many author ities maintain that Daguerre took unfair advan tage of his partner, and published, as his own, processes for which Niepce should have had equal credit. However this may be, it is certain
that Niepce died in 1833, some years before Daguerre produced any pictures by the method that now bears his name; and it would appear that Daguerre is at least entitled to the sole credit for the discovery of inercury as a de veloping agent. The way in which this discovery came about is of deep interest, and was cited by Professor Liebig as one of the finest ex amples of the inductive method of reasoning. Daguerre had discovered that iodide of silver is affected by light, and he had repeatedly iodized silver plates and exposed them in his camera, with the result that feeble images were obtained. He was filled with hope that some way might be found to intensify these images, but he worked for years without success, and Niepce died with a feeling of regret that they had wasted so much time upon a method that was apparently incapable of yielding the results that they sought. On one occasion, after the death of Niepce, Daguerre removed one of his old plates from a closet in which it had been stored, and was about to repolish it and use it over again in a new experiment, when he observed that the view to which it had been previously exposed, and which showed but faintly when he had put the plate away, was now strong and clear.
Without disturbing anything in the closet, he prepared a new plate, sensitized and exposed it as before, and placed it in the closet for a similar time. The same intensification of the image was observed. He concluded that the developing agent that he had sought so inde fatigably was present in his closet, but he had no idea what it could be. To identify it, he prepared and exposed plate after plate, each time leaving the plate where the first one had stood, but each time removing one article from the closet. The pictures still developed, even when the last thing had been apparently re moved. He found, however, that some mercury had been spilled in the closet, and being driven to the conclusion that this was the mysterious agent sought, he tried it, and his mercury de velopment process was the result.