82 J. Waterhousc — The Application of Flio to graphj [No. 2, 



Asphaltum methods. — In 1852, MM. Lemercier, Lerebours, Barresvvil 

 and Davanne, proposed a method of litho-photographj, in which a stone was 

 coated with a solution of bitumen in ether, exposed to light under a reversed 

 negative, and developed with ether, which dissolves the parts not affected by 

 the light, while the exposed parts being insoluble remain and form the image. 

 (Benzole, chloroform or turpentine may also be used instead of ether). 

 After development the stone was prepared with acid and gum and inked 

 in the same way as an ordinary lithographic drawing.* 



Since then many other similar asj)haltum processes have been proposed 

 and have been worked with great success, both for subjects in line and half- 

 tone ; but, owing to the length of exposure required and the uncertainty of 

 the results, this process is not well ada|)ted for general use, and has, I 

 believe, been almost abandoned in favour of the collochromate methods. 



Coll o-chr ornate Processes. — Paul Pretsch, whilst working out his photo- 

 galvanographic process, hereafter to be described, discovered that if a 

 mixture of gelatine and bichromate of potash be spread upon a suitable 

 support and when dry exposed to light, then again moistened and inked 

 in with a roller charged with printing ink, the ink would only take upon 

 the parts altered by the light, and thus impressions could be obtained by 

 transferring the design to zinc or stone. 



Pretsch does not seem to have made any practical use of this discover}^, 

 but shortly afterwards, in 1855, Poitevin independently w^orked out a 

 photolithographic process on the same principle, which has been the founda- 

 tion of all the present processes of photolithography and photocollotype 

 and is worked to the present day for the reproduction of the Belgian 

 topographical maps. Poitevin impressed his photographic image direct 

 upon the stone and not by transfer. 



The first practical transfer process of photolithography seems to have 

 been suggested by Asser, of Amsterdam, early in 1859. He coated unsized 

 paper with starch, and then floated it on a strong solution of bichromate of 

 potash. When dry it was exposed to light under a well intensified nega- 

 tive. The print was next heated with a flat iron, then moistened and inked 

 in with transfer ink, by means of a roller, and thus an impression was obtain- 

 ed which could be transferred to stone or zinc.f 



The next transfer processes were the Southampton process of photo- 

 zincography, which was founded on Asser's, and Mr. Osborne's process of 

 photolithography. These two processes, though quite independent one of the 

 other, were identical in principle and almost so in details ; the only difference 

 being that Mr. Osborne added a certain proportion of albumen to the mixture 

 of gelatine and bichromate and then treated his prints with boiling water, in 



* See Davanne, Chimie Photographique, p. 456. 

 t Thotographic News, Vol. Ill, p. 146. 



