The advance of photography : its history and modern applications (1911)

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14 THE ADVANCE OF PHOTOGRAPHY this way photography became especially an art of portraiture. It made the taking of portraits its principal means of support, and in two years there were daguerreotypists in all the capitals of Europe. In America a painter, Professor Morse, afterwards the inventor of the Morse telegraph, was the first to prepare daguerreotypes ; and his coadjutor was Professor Draper. The Daguerreotype. — Let us now consider more closely the process employed in producing daguerreotype plates. A silver plate, or in the place of it a silver-plated copperplate, serves to receive the image. It is rubbed smooth by means of tripoli and olive oil ; and then receives its highest polish with rouge and water and cottonwool. It is only a perfectly polished plate that can be used for the process. This burnished plate is placed with its polished side downward upon an open square box, the bottom of which is strewn with a thin layer of iodine. This iodine evaporates, its vapour comes into contact with the silver, and instantly combine with it. By this means the plate first assumes a yellow straw-colour, next red, then violet, and lastly blue. The plate, protected from the light, is then placed in the camera obscura, where the image on the ground-glass slide is visible, and " exposed " for a certain time. It is afterwards brought back into the dark, and put into a second box, upon the metal floor of which there is mercury. This mercury is slightly warmed by means of a spirit lamp. At first no trace of the image is visible on the plate. This does not appear until the mercury vapour is condensed on those parts of the plate which have been affected by the light ; and this condensation is in proportion to the change which the light has caused. During this process the mercury is condensed into very minute white globules, which can be very well discerned under the microscope. This operation is called the development of the picture. After the development the remaining iodide of silver,