1878.] to fJfc JReprocluction of Maps and Flans. 81 



an ordinary engraving or manuscript, it is exposed in a closed box to the 

 vapour o£ anilin, which developes a greyish image. The print is then fixed 

 by merely washing with water. As a positive original yields a positive 

 print, maps or drawings may be copied without the necessity o£ making a 

 negative by means of a camera, which is a great recommendation in 

 certain cases. The process has hitherto been worked only by the inventor 

 and his licensees and has not come into general use. 



V. PhOTOLITHOGEAPHT and PHOTOZINCOaEAPHT. 



In all the processes noticed in the last section, it is necessary to repeat 

 the printing operation by exposure to light for every print produced. The 

 rate of printing will consequently be more or less dependent on the sensi- 

 tiveness of the paper, the strength of the light at the time of exposure and 

 the state of the weather ; the printing operations can, moreover, only 

 be carried on during the few hours of daylight. In the photo-mechanical 

 processes, now about to be described, these grave disadvantages are obviated, 

 and, once the photographic image has been produced upon the printing 

 surface, prints may be made in any numbers, quite independently of light 

 or weather. 



The simplest and most generally useful of these mechanical processes 

 is j^hotolithography, or the analogous photozincography, the principal 

 difference between the latter and the former being merely the substitution of 

 a thin smooth plate of grained zinc for the thick heavy lithographic stone. 

 For maps of large size, zinc is certainly the most suitable and offers in other 

 respects all the advantages of stone, but the latter being better known is 

 generally preferred for ordinary work of moderate size. 



In ordinary lithography, the image may be produced on the stone or 

 zinc either by transfer from a drawing on paper with the solution of resi- 

 nous soap known as ' autographic ink', or by drawing direct on the stone with 

 a similar ink or crayon ; so in j)hotolithography there are two similar 

 methods of obtaining the photographic image — either by transfer from a 

 photographic print in fatty ink — or by impressing the image direct on tlie 

 stone, by applying a photographic negative on a suitable coating sensitive 

 to light and removing by means of a solvent the parts unaltered by liglit. 

 The transfer method being the most convenient is the one in general use. 



The first photolithographic process on record is that proposed by 

 Jobard, of Brussels, who, in 1839, obtained lithographic proofs from stone 

 or zinc plates that had been treated with iodine or bromine. This process 

 never came into practical use and has been quite superseded by two distinct 

 methods — one dependent on the alterability of asphaltum under the influ- 

 ence of light — the other on the reactions of. the alkaline bichromates upon 

 gelatine and otlicr colloid substances. 



