A Manual Of Photography

PICTURES ON PORCELAIN TABLETS. 31 brown or sepia tint. Under this treatment the picture dimi- nishes in size, insomuch that if it were previously cut in two, and one part submitted to the potash process, and the other not, the two halves, when afterwards put together, would be found not to correspond. The advantages of this process for re- moving any iodine which, even after fixing with the hyposulphite, remains in the paper, is great, and it will tend much to preserve these beautiful transcripts of nature. The patentee then claims as an improvement the use of varnished paper, or other trans- parent paper, impervious to water, as a substitute for glass, in certain circumstances, to support a film of albumen for photo- graphie purposes. A sheet of writing-paper is brushed over with several coats of varnish on each side: it thus becomes extremely transparent. It is then brushed over on one side with albumen, or a mixture of albumen and gelatine, and dried. This film of albumen is capable of being rendered sen- sitive to light by exposing it to the vapour of iodine, and by following the rest of the process indicated in the preceding section of this specification. The advantages of using varnished or oil paper do not consist in any superiority of the images over those obtained upon glass, but in the greater convenience of using paper than glass in cases where a large number of pictures have to be made and carried about for considerable distances: besides tins, there is a well-known kind of photographic pictures giving panoramic views of scenery, which are produced upon a curved surface by a movement of the object-glass of the camera. To the production of these images glass is hardly applicable, since it cannot be readily bent to the required curve and again straightened ; but the case is met by employing talc, varnished paper, oiled paper, &c. instead of glass. It will be seen that tlie varnished paper acts as a support to the film of albumen or gelatine, which is the surface on which the light acts, and forms the picture. The next improvement consists in forming photo- graphic pictures or images on the surfaces of polished steel plates, lor this purpose, one part (by measure) of a saturated solution of iodide of potassium is mixed with 200 parts of albumen, and spread as evenly as possible upon the surface of a steel plate, and dried by the heat of a gentle fire. The plate is then taken, and, whilst still warm, is washed over with an alcoholic solution of gallo-nitrate of silver, of moderate strength. It then becomes very sensitive, and easily receives a photographic image. If the Plate be cold, the sensibility is considerably lower. The image obtained is fixed by washing with hyposulphite of soda, and finally with water. The print adheres to the steel with much enacity, and forms a process very useful to engravers. With