102 J. Waterhouse — The Application of Fhotograpliy [No. 2, 



may be obtained without difficulty, and the expensive hand- work of the 

 engraver in biting in or finishing may to a great extent be dispensed with. 



The processes of photographic engraving that have been proposed from 

 time to time for producing incised images on metal plates capable of being 

 printed in the copper-plate press, are very numerous. I shall, however, con- 

 fine myself to those which have been most successfully worked and of 

 which the details have been more or less fully published. Further informa- 

 tion on the subject will be found in the special works referred to in the 

 footnotes, and also in Hammann's ^' Des Arts Qrapliiques destines a 

 multiplier par V Impression^'' and A. Martin's " Handhuch der Email- 

 pliotograpTiie und der JBliototypie oder des Lichtdruckes,'''' which both give 

 very complete resumes of the early progress in this branch of photography, 

 with details of many of the processes. The Photographic Journals and the 

 Patent Office records may also be consulted. 



The principal methods of obtaining an incised image on a metal plate 

 by means of photography are : 



1. Obtaining a photographic image on a metal plate coated with 

 asphaltum and then etching or 'biting in' with acid. 



2. Obtaining a photographic image in gelatine on a metal plate and 

 etching the latter with some substance that will not attack the gelatine. 



3. Obtaining an image by the direct action of light on a metal plate, 

 as in the Daguerreotype process, then forming a metallic reserve to protect 

 either the lights or shadows of the image and etching with a suitable 

 mordant, 



4. Electrotyping from a relief obtained by the swelling or partial 

 solution of a chromated gelatine film, either directly or by the intervention 

 of a cast in wax or plaster. 



5. Electrotyping from a relief in insoluble gelatine obtained in the 

 same way as in the ' Autotype' or Pigment-printing process. 



6. Electrotyping from a leaden plate on which an image has been im- 

 pressed from a gelatine relief, as in the Woodbury-type process. 



7. Electrotyping from a relief obtained directly on a collodion 

 positive cliche. 



It will be seen that these methods divide themselves into two principal 

 groups of etching and electrotyping processes. 



Etching processes with Asphaltum. — "We have already seen that Niepce 

 in his experiments to find a substitute for lithography, made use of the pro- 

 perty possessed by bitumen of Judaea, or asphaltum, of becoming insoluble in 

 oil of lavender and other solvents, after exposure to the action of light, to 

 obtain photographic images on metal plates which were then bitten in with 

 acid, so as to form engraved plates, usually copies of engravings, though 

 he also Obtained images from nature. 



