86 J. Waterhouse — The AppUrntlon ofPlotogrufliy [No. 2, 



after the preliminary coating has been given to the paper. The gelatine 

 may also be hardened with alum or chrome alum. 



It has been found that this method has the advantage that a base of 

 hard insoluble gelatine remains on the paper and retains the finest lines, 

 while the fresb and easily soluble final coating preserves the clearness of 

 the ground. It is necessary that the underlying gelatine should be 

 thoroughly hardened, otherwise the transfers stick to the zinc plate in 

 transferring, and are difficult to remove ; the soft gelatine is also liable to 

 spread ov«r the lines and prevent their transfer. 



Another advantage is that warm water is not required for washing the 

 prints, and the ink is not so liable to become pasty as in the usual mode of 

 working. The lines are found to keep crisp and the spaces between them 

 free from scum, thus giving clearer and sharper transfers. 



Mr. Herbert Deveril, Government photolithographer in New Zealand, 

 found that, in working Osborne's original process of photolithography, 

 which is still generally used in the Australian Colonies, great inconvenience 

 arose from the use of boiling water to coagulate the albumen added by Mr. 

 Osborne to the gelatine in order to produce an insoluble surface with a 

 * grip' on the stone. He has therefore substituted the following method of 

 producing his transfer prints. Paper is first coated with gelatine to which 

 a small proportion of chrome alum has been added. This is allowed to dry 

 and is then sensitised in a solution of bichromate of potash. The prints 

 are exposed and inked as in the Southampton process, and are washed off 

 in cold water.* Mr. Deveril claims for this method the further advantage 

 that the sensitive paper can be kept in good condition for a long time. 

 The keeping properties of papers coated with gelatine and bichromate are, 

 however, very dependent on climatic conditions. The results which I have 

 seen by the process are exceedingly good. 



A method of photolithography by transfer which yields excellent results 

 in line, and even reproduces half -tones fairly well, is a modification of Asser's 

 process, iuA^ented by Mr. Toovey, of Brussels, who coats paper with a solu- 

 tion of gum arable mixed with bichromate of potash, and after exposure to 

 light under the negative in the usual way, places the transfer-print face 

 downwards on the stone with several thicknesses of wet blotting paper over 

 it, and leaves it under pressure for some hours in a powerful press. 



The gum on the parts not exposed to light being soluble is forced into 

 the stone and prepares it, while the lines being hardened and rendered in- 

 soluble leave the stone quite free from gum and ready to take j^rinting ink 

 from a roller when passed over them, thus producing an image which may 

 be printed from as soon as the soluble bichromate salt has been washed out, 

 because the bichromated gum is a most powerful preparation for the stone 

 * ' Fhotographic News,' Vol. XIX, p. 585. 



