" The stone should now be placed in the hands of the printer, who will etch it, as he would a chalk drawing, using perhaps a little less acid ; he will then cover it with gum in the u.sual way, and the next day it will be ready for printing. A great deal depends, as in lithography generally. on its being put into the hands of an ex perienced picture lithographer. The stone admits of no re-touching, but letters can be added with the use of lithographic ink, as required. I may add that the impressions always look best on India paper, on account of the tint.
" I have never worn out a stone. Of each of the two specimens exhibited 500 copies have been thrown off and no difference could be seen between the first and the last." The second process was patented in England by M. Poitevin, of Paris, on April 15th, 1856. The specification of the patent gives the following particulars :— " I print photographically, with ink of a greasy nature, upon paper, lithographic stone, metal, glass, wood, or other suitable ma terial, in the following manner :— " I apply upon the surface which is to receive the design one or more layers or films of a mixttut of equal parts of a concentrated solution of albumen, fibrine, gum arabic, gelatine, or similar organic substanc,e, and a concentrated solution of a chromate or bichromate of potass, or of any base which does not precipitate the organic matter of the first solution. This single or compound layer or film is then dried, if the photographic impression is to be produced by contact ; or it may be used in a moist state when the photographic impression is to be produced in the camera obscura. In producing
the impression by contact, the surface is covered with a photo graphic negative picture, or an engraving, or other transparent or partially transparent object or screen, and then exposed to light as in the ordinary photographic process. After a sufficient exposure, if the surface has become dry, or has been used in a dry state, it is moistened with water by means of a sponge, and while moist the greasy ink or matter is applied to the surface by a ball or dabber, or by a roller or press, or otherwise, and it will be found to adhere to those parts only which have been affected by the light. Thus, if the screen employed be a negative, having the lights and shades reversed, the piint will be a positive, with the lights and darks cor rect ; and if the screen be a positive, the print will be a negative. The print may be retained on the surface upon which it is first pro duced, or it may be transferred or printed upon paper, or other suitable material, and the operation repeated. I thus obtain a de sign upon lithographic stone, or other suitable material, from whicli I am enabled to multiply impressions by the method of lithographic printing by inking the moistened surface with a greasy ink."