1878.] fo the Reproduction of Maps and Plans, 109 



* grain' by interposing between the cliche and the gelatine film a fine net- 

 work or an impression of an engraved or roulette tint ; but these artificial 

 grains have a disagreeable effect, and the methods seem to have fallen into 

 disuse, except for line- work and photo-typography, which will be described 

 further on. Avet's process is, however, I believe, still in use for producing 

 the maps of the Italian Surveys. 



Geymefs method. — The fourth method, that of electrotyping from a 

 gelatine relief obtained by the pigment-printing process, is somewhat similar 

 in principle to Placet's process, but as there are important differences and 

 the process appears likely to prove of some utility, it may well be treated 

 separately. 



According to M. Geymet, who has very fully described the process and 

 all the manipulations of preparing and electrotyping the reliefs in his 

 *' Gravure Heliographique^'' it was the invention of M. Audra, a French 

 amateur. 



Pigmented gelatine tissue is sensitised and exposed to light exactly in 

 the manner described at p. 78 for the ' Autotype' process. It is transferred 

 to a smooth glass or a polished copper plate, developed in warm water, and when 

 dry is metallised and electro typed. If the subject is one in line or dot only 

 the above operations are sufficient, but if the subject is a photograph from 

 nature, or any other with gradation of shade, it is necessary to obtain a 

 * grain', and this M. Geymet does by taking a copper-plate with its surface 

 grained or engraved with a ruled or roulette tint, inking it up in the ordi- 

 nary way. and then covering it with a coating of transfer collodion. When 

 dry, the film of collodion is stripped off the plate and carries with it the 

 impression of the grain. This film is then placed between the cliche and 

 the sensitive gelatine film and serves to break up the shadows in the more 

 transparent parts of the cliche. 



A similar process has been used at the Depot de la Guerre, in Belgium, 

 for the reproduction of maps,* 



Last year, whilst making some experiments on this process, I succeed- 

 ed in obtaining the necessary ' grain' by chemical means which produce a 

 finer and less artificial effect, and I have also made a few other modifica- 

 tions in the process, which may be worth recording at length. 



A piece of the ordinary autotype tissue is sensitised in a 5 per cent 

 solution of bichromate of potash. When dry, it is exposed to light under a 

 reversed negative and then transferred in cold water to the surface of a well 

 polished copper plate and squeegeed down into close contact with it. In 

 order to prevent subsequent adherence to the newly deposited copper in the 

 electrotyping bath, the copper plate is silvered by rubbing it with a little of 

 the following solution mixed with tri]K)li. 



* Sec Maes and Ilaimot ' Truitc dc Topographic^ ij'c'-/ P- ^^0- 

 15 



