1878.] to the Beproduction of Maps and Plans 105 



potash in contact with organic matter under the influence of light. Talbot 

 found that by the action of light, a dried film of gelatine mixed with an 

 alkaline bichromate became impermeable to certain fluids in proportion to 

 the intensity of the action of the light upon it. He coated steel plates 

 with a thin film of gelatine and bichromate of potash, and after exposure 

 to the light under a photographic positive, he etched the image so produced 

 with a solution of bichloride of platinum which, penetrating the unaltered 

 gelatine in the parts protected from the light and attacking the underlying 

 metal, produced the shadows of the resulting picture. Some very promising 

 results were obtained in this manner, and great expectations were enter- 

 tained of its utility in producing engraved plates for book illustration 

 and other purposes. These hopes, however, have not been fulfilled and the 

 process, though remarkable as the first of the many valuable methods of 

 photographic press-printing dependent on the use of gelatine and the 

 alkaline bichromates, has inherent defects and difiiculties which seem to 

 render it of little practical value.* 



M. Baldus has successfully employed a modification of the photo- 

 glyphic process for line- work. f He coats a copper-plate with gelatine and 

 bichromate and exposes it under a negative or a positive, then etches in a solu- 

 tion of perchloride of iron, which attacks the copper in all the parts not acted 

 upon by the light, and thus a first relief is obtained. As this relief is not 

 sufiicient, the plate is inked in with a printing roller|^hen the ink attaches 

 itself to the parts in relief and protects them from the action of the etching 

 liquid. This procedure is repeated till the desired effect is produced. If a 

 negative is used an incised plate is obtained, which may be printed in the 

 copjDcr-plate press. If a positive is used the image is in relief and suitable 

 for being printed with type. I have found that the reliefs obtained in this 

 way are exceedingly sharp, though the gelatine films will not stand the 

 action of the etching fluid for very long. 



Messrs. Leitch and Co., of London, have lately introduced a similar 

 process, called by them ' Photogravure.' It appears to be due to M. Garnier, 

 who has had great experience in these processes and produced some very 

 fine results. The method of working is a secret, but it is said that a metal 

 plate is coated with a sensitive composition capable of resisting the action 

 of acids. The photographic image is impressed on the sensitive surface 

 through a negative and is then etched with perchloride of iron. The etch- 

 ing is said to be to a certain extent automatic, that is to say, the etching 

 action on the lines ceases at different periods in proportion to their fineness. 



* A full description of Talbot's process, with specimens, will be found in the 

 appendix to the Enghsh translation of Tissandier's ' History and Handbook of Fhoto- 

 graphy^'' edited by J. Thomson. 



t See the above work, p. 207. 



