1857.] and Electricity to the production of Engravings. 345 



varnish would protect the metal from the acid over the lights of the 

 picture ; while the shadows, represented by the bare metal, would 

 be bitten in the manner common to all etching processes. On now 

 removing the protecting varnish, the plate could be inked and 

 printed from by the common copper-plate printing press. Such 

 are the essential details of the first of the photographic engraving 

 processes. The specimens on the table were presented in 1827 to 

 Mr. Bauer, late of Kew, by Nict^phore Niepce, who for a short 

 time resided at Kew, on a visit to a brother in infirm health. 

 Niepce prepared a statement regarding his invention, for present- 

 ation to the Royal Society ; but as he at that time kept his process 

 secret, his manuscript was not published. Niepce appears to have 

 returned to France, disappointed at his ill-fortune. 



Niepce's bitumen process was improved by his nephew, 

 M. Kiepce de St. Victor, who has published a treatise* on it, 

 giving the necessary minute instructions. The main features do 

 not differ from those above given, though greater sensitiveness and 

 perfection have been obtained. MM. Mante, Belloc, and Negre, 

 MM. Barreswil Davanne, Lerebours, and Lemercier have also 

 advanced the bitumen process : the latter gentlemen having applied 

 it to lithographic purposes. The process is still under trial ; but 

 the difficulties of obtaining a constantly uniform result at present 

 stand in the way of its general adoption. It still deserves a thorough 

 investigation. 



The second method of producing photographic engravings is 

 founded upon certain properties possessed by the Daguerrean 

 image. It is found that a daguerreotype unfixed by gold is acted 

 upon by nitric acid in its shadows, while the lights long resist 

 the biting action of the acid. This is explained by assuming that 

 the shadows are of pure silver, and that the lights consist of mercury 

 — the acid attacking the silver by preference. The fact is, that an 

 etching is obtained by merely leavng idiluted nitric acid in contact 

 with the plate. The etched plate is then inked and printed from 

 as in Niepce*s case. Dr. Donne, of Paris, appears to have been 

 the first to devise this method. Dr. Berres, of Vienna, also used 

 nitric acid for this purpose ; but the action is not easily controlled, 

 and this form of the process has fallen into disuse. In 1842, 

 Mr. Grove published in the Philosophical Magazine a method by 

 which Daguerre's images can be engraved by the chlorine evolved 

 by voltaic action, when the daguerreotype plate is made the positive 

 terminal of the battery, and immersed in diluted hydrochloric acid ; 

 the negative wire being terminated by a plate of platinum, which 

 was placed opposite and parallel to the photographic image. This 

 process is much more under control than the last. [Prints from 

 plates so engraved in 1842 were on the table.] This process is also 



♦ Traitc Pratique de Gravure H61iographique, par M. Niepce de Saint- 

 Victor. Paris, .Tuin 1856. 



