1S7S.] to ilic Btiprodiiction of Maps and Plans. 77 



the negative, representing the lines of the subject, it renders the colloid 

 coating insoluble throughout the thickness of the coloured film, so that 

 the lines withstand the solvent action of the warm water, which entirely 

 removes the rest of the coloured film from the ground and parts which have 

 not been influenced at all by the light. If, however, instead of a negative 

 of a line subject, on which the lines are transparent and the ground opaque, 

 we take a negative of a subject in half tones, possessing various degrees of 

 translucency in the lights and shadows of the picture, and make a print 

 from it on a piece of the pigmented paper, we shall find that the light will 

 only be able to penetrate through the entire thickness of the colloid film 

 in the deepest shadows, represented, as before, by nearly clear glass ; in 

 the darker half-tones it will penetrate nearly through the coating ; in the 

 middle tones about half-way through, and in the lightest tones tlie light will 

 be able to act only on the surface of the gelatine. We shall therefore 

 have a print with an insoluble surface of varying depth, and underlying 

 this a more or less soluble layer ; it will thus readily be understood that 

 when exposed to the action of warm water this layer will dissolve and carry 

 away with it the partially insoluble surface-film forming the half shades of 

 the picture, leaving only the stronger shades and giving a rough, hard, and 

 unfinished appearance to the print. 



For a long time this difficulty proved a stumbling-block in the way 

 of the progress of permanent printing and gave the silver-printing pro- 

 cesses a supremacy of which it has now become difficult to deprive them. 

 The Abbe Laborde was the first to see the necessity for adopting the 

 principle of exposing on one side and developing on the other. Blair, 

 Fargier and Swan applied this to the carbon process, and the latter finally 

 succeeded in introducing a practical method of pigment-printing applicable 

 to the same class of subjects as silver-printing. Swan prepared a tissue by 

 coating paper with a thick layer of gelatine mixed with bichromate of potash 

 and coloured with any suitable pigment. After the exposure to light 

 the gelatinous surface of the tissue was caused to adhere closely to a second 

 piece of paper coated with india-rubber. The whole being immersed in 

 hot water, the paper on which the gelatinous layer was originally supported, 

 became loosened and could be removed, allowing the hot water to gradually 

 dissolve away the unaltered and soluble gelatine. In this manner the 

 exposure to light takes place on one side of the gelatine film, while the 

 washing away of the superfluous gelatine is effected from the other, or 

 unexposed side, without disturbing in any- way the exposed parts of the 

 film, and thus the most delicate shades in the half tones are perfectly 

 preserved. Since its introduction by Swan this process has been much 

 improved by Messrs. J. R. Johnson, li. Sawyer and other members of the 

 London Autotype Company which acquired Swan's patents, and under 

 11 



