Osborne.] 4:S4: [December. 



Having proceeded thus far, it is necessary to prepare the sensitive 

 surface upon which the positive print from this negative is to be made. 

 All the processes of which anything was known or published up to 

 my time, were based upon the idea that the surface of the stone 

 should be made sensitive to the action of light, and that the photo- 

 graphic picture formed thereon should possess the necessary and 

 peculiar, lithographic properties. I deviated from my predecessors 

 in this respect, and struck out a new course, which at once gave 

 superior results. This consisted in sensitizing a sheet of paper in 

 such a way as to make it fulfil similar conditions; and having pro- 

 duced upon it a photograph in lithographic ink adapted for the pur- 

 pose, the same is transferred to stone by a well-known lithographic 

 operation, and printed in the ordinary way. 



To effect this object, a sheet of paper of the best quality is pre- 

 pared with a solution of gelatine and bichromate of potash in water, 

 to which a quantity of albumen has been added. This mixture is 

 poured into a long narrow trough, and one side of the paper is covered 

 with it, by drawing a sheet over the fluid in the trough, while an 

 assistant presses it into contact with the same by means of a piece of 

 wood of suitable form. 



The paper thus coated is carefully dried in the dark, and upon it 

 a positive print from the negative above mentioned is printed by light 

 in the manner practised by photographers. The result is that a brown 

 picture makes its appearance upon the clear bright yellow of the 

 paper, identical in every respect with the original which was copied, 

 unless perhaps a reduction or enlargement in size may have been 

 decided on. It is not however the change in color which makes this 

 picture valuable for photo-lithographic purposes, but rather the altera- 

 tion in the chemical and physical properties of the organic substances, 

 which form the superficial coating upon the sensitive paper, the na- 

 ture of which will be understood when the following operations are 

 described. These are technically known as "blacking," "swimming," 

 and " washing off." Blacking an exposed positive has for its object 

 the distribution of an even coating of lithographic transfer ink over 

 its surface. Such an ink is essentially composed of greasy or resinous 

 substances fused together, and blackened with lampblack. For our 

 present purpose it is distributed with the printing roller over the 

 surface of a stone in the press, and upon it the exposed positive print 

 is laid, with the photographic picture downwards, and in contact 

 with the ink. After passing both stone and paper through the press, 

 and separating them, the latter will be found to have brought away 



