Ography
Take a plate of glass, and spread on it ordinary glue, to which bichromate of potash and a small quantity of nitrate of silver has been added. For instance, take 2 or 3 solutions of glue, into one of which put a little nitrate of silver, into another bichromate of potash, and into another iodide of potassium. The silver and the iodide are for the purpose of producing a little iodide of silver on the sensitive film, so as to produce on the picture that grain which is necessary for holding the ink in the process of printing. Take the photographic picture obtained by any of the customary processes, and place it on the sensitive plate thus In the course of a short time (all those parts which are dark in the photograph, protecting the plate from change, and all those which are white, allowing the sunlight freely to pass through and the change to take place), we have a combination of bichromate of potash and gelatine in two different states, one soluble and the other insoluble. Consequently, when the plate is then put into water all the parts which remain soluble are dissolved out, whilst the other parts remain as they were, and we have the picture produced not only in different lights and shades, but also in different depths, the solution being eaten into by the process. When the plate is prepared to this point, there is poured upon it a preparation of guttapercha, which, being kept under pressure for a short time, receives the reverse image of the photographic picture. This is now prepared for the voltaic battery by being simply rubbed over with fine black lead, and it being placed in connection with the trough, copper is precipitated on the plate, which receives an image the reverse of the mould. Then by the ordinary electrotype process another plate may be obtained, from which copies may be printed. 2