1878.] to the Beprodioction of Maps and Flans. 83 



order to congulate the albumen and leave a slight coating of it on the 

 paper, so as to obtain a 'grip' on the stone during the j^rocess of transfer.* 



At the Ordnance Survey Office, Southampton, and at the various pho- 

 tographic offices in India, in which the process has been introduced from 

 Southampton, photozincography is used, with the best results ; but in 

 Australia, America and the Continent of Europe photolithography is more 

 usual and it is also used at Madras. 



These processes have occasionally been used with fair success for the 

 reproduction of shaded maps, architectural views and other subjects in half- 

 tones, but they are not by any means suitable for such subjects, and are 

 best adapted for the reproduction of maps and drawings boldly executed in 

 dot or line alone. 



They may also be used for copying prints or engravings of all kinds 

 on the same, larger or smaller scales, but engravings, and even many 

 lithographs, are generally more or less unsuitable for the purpose. The 

 best results are obtained from original drawings specially prepared to suit 

 the requirements of photographic reproduction, in strict accordance with 

 the rules already given. 



The following outline of the Southampton method will give an idea of 

 the operations. Like the pigment-printing process, already described, it 

 depends upon the property possessed by a dried layer of gelatine and other 

 colloids, when mixed with an alkaline bichromate, of becoming insoluble and 

 repellent of water under the influence of light. The procedure, too, is much 

 the same as in the siniple pigment-printing process, except that, instead of 

 the fatty ink which forms the image on the photo-transfer print being 

 mixed with the gelatine, it is applied to the surface of the print after 

 exposure to light. The inked print is then washed in hot water, by which 

 the colloid coating in the unexposed parts is dissolved and carries away 

 with it the superfluous ink not retained by the lines forming the image. 



The negatives are obtained by the methods already described appli- 

 cable to the reproduction of subjects in line. 



Having obtained a suitable negative, the next operation is to produce 

 from it a photograph in greasy ink which may be transferred to zinc or 

 stone. 



To prepare the sensitive paper, a sheet of bank-post paper is coated 

 twice with a mixture of G parts gelatine and 4 parts bichromate of potash, 

 dissolved in 100 parts of water, dried in the dark and glazed to give it a 

 smooth surface. It is then exposed to the light under a negative for one or 

 two minutes in the sun, or until the finest lines are distinctly visible. When 

 sufficiently exposed, which may be ascertained by the whole of the detail 

 appearing in brown upon a bright yellow ground, the print is taken out of 

 * Fhotographic Neivs, Vol. IV, p. 374. 



