112 J. W:>terlioiise — The Application of Vliofograply [No. 2, 



mixture of" gelatine and bichromate of potasli, containing a quantity of fine 

 emery, powdered glass or charcoal is poured over it and allowed to * set'. 

 The gelatine film is then dried and taken from the glass, and the collodion 

 side exposed beneath a negative. After a sufficient exposure, it is tempo- 

 rarily attached, on the collodion side, with india-rubber solution, to a sheet 

 of glass and washed in warm water. 



The resulting granular image is then pressed into a sheet of soft metal 

 by means of the hydraulic press. The soft metal plate has an electrotj'pe 

 made from it in copper, and another plate, subsequently covered with a 

 coating of iron, is again made from this to serve as the printing-plate, the 

 first copper plate being kept as a reserve.* 



Mr. Woodbury also describes another method which in some respects 

 resembles Geymet's, before described. 



Paper is successively coated with three or more mixtures of gelatine, 

 bichromate of potash and some granular substance in different degrees of 

 fineness — first with the coarsest and lastly with the finest. When dry, the 

 tissue is exposed under a negative, transferred under water to a finely polished 

 plate of zinc or steel, then washed in warm water as usual, and when dry is 

 ready for pressure into the soft metal block. In this case, the light tones 

 are composed of the finest grains and the shadows of the coarsest. 



M. Rousselon, .the manager of Messrs. Goupil's photographic works 

 at Asnieres, near Paris, has obtained engraved plates with remarkably good 

 half-tones by a process somewhat similar to the Woodbury-type, which is 

 also largely worked by Messrs. Goupil. The peculiarity is in the grain, 

 which is obtained by the addition to the sensitive mixture of gelatine and 

 bichromate of some substance which has the proj^erty of causing the film 

 to become granular under the influence of light, the granular effect being 

 increased in proportion to the intensity of action of the light. The other 

 operations are the same as in the first of Mr. Woodbury's processes just 

 described. The details of this process are a secret, but it is said that the 

 substance used for producing the grain is chloride of calcium. 



I am not aware of either Woodbury's or Rousselon's processes being 

 utilised for the reproduction of maps, but in certain cases they could, no 

 doubt, be usefully employed. The only difficulty seems to be that an 

 immensely powerful hydraulic 2:>ress is required for large subjects. 



^lioto -mezzotint. — The Editor of the ^British Journal of PJiotography ' 

 has lately suggested a process of photo-mezzotint engraving founded on the 

 ' dusting on' or * powder' process, already alluded to.f 



A polished steel plate is thinly coated with — 



* ^British Journal Photographic Almanac,'' 1872, p. 40. 

 t ^British Journal of Fhotography^^ Vol. XXIV, p. 170. 



