1878.] to the Beprodurtion of Maps and P/ans. 99 



VII. WOODBUET-TTPE. 



We have already seen that the great drawback to the production of 

 photographs in printing ink with continuous gradation of shade, by eitlier 

 photography or photo-engraving, is the necessity for breaking up the 

 continuity of gradation by a more or less marked ' grain', and that this 

 difficulty has been overcome by the photocollotype processes. 



By a very ingenious process, invented in 186i, Mr. Walter Woodbury 

 succeeded in solving the problem in another way, and, by a mode of operation 

 analogous to ' Nature-Printing', has been able to produce absolutely per- 

 manent prints with such perfect photographic gradation, combined with 

 the most exquisite transparent delicacy and richness of tone, that none 

 but the initiated would know that they were not the ordinary silver 

 prints. 



A tissue is first made by coating a tough film of collodion with a 

 moderately thick even layer of gelatine and bichromate of potash, slightly 

 coloured in order to see the progress of the development. When dry, 

 the tissue is laid collodion side next to the negative film, and exposed to light 

 proceeding from one direction only, in order to prevent diffused rays acting 

 through the thick gelatine coating and so blurring the image. This tissue of 

 gelatine and collodion is then temporarily attached to a glass plate and 

 treated with hot water, very much in the same way as in the pigment 

 printing process already described. The whole of the gelatine upon which 

 the light has not acted, and which therefore remains soluble, is dissolved 

 away, leaving an image in relief, the highest parts of which represent the 

 deepest shadows of the picture, while the parts intervening, down to the 

 lowest, represent the intermediate gradations between the deepest shadows 

 and the highest lights. 



When dry, the gelatine composing this image will be quite hard and 

 capable of resisting the heavy pressure required to indent it into soft metal, 

 without itself being injured. 



The tissue bearing the image having been stripped from the temporary 

 support, is laid face downwards on a sheet or block of lead or type-metal, 

 about ^ of an inch thick, between two finely surfaced steel plates and 

 submitted to the pressure of a very powerful hydraulic press. The pro- 

 minent parts of the relief are thus forced into the soft metal and produce a 

 mould the deeper parts of which re2:)resent the shades and the shallower the 

 lights of the picture. As the relief obtained from gelatine and bichro- 

 mate of potash alone will impart to this mould a smooth surface without 

 grain, such plates could not bo printed with printers' ink, like a coj:)per- 

 plate engraving. Mr. Woodbury therefore uses a semi-transparent ink 

 consisting of gelatine coloured with any suitable pigment. 



