CHAP, viii.] HELIOGRAPHY PHOTOLITHOGRAPHY. 317 



(albumen, gum, gelatine) impregnated with alkaline bichromates, of 

 being acted on by light and becoming insoluble. 



M. Poitevin's invention was not so successful as he expected : only 

 the strongest parts of the image came out well, the Half tints were 

 carried away, because, as M. Laborde discovered, the impressed coating 

 was very thin and the gelatine coating underneath dissolved in water 

 and carried away with it the lightest parts of the image. A French 

 photographer, M. Fargier, found a means to remedy this inconvenience 

 by developing the proof on the side of the gelatine opposite to the im- 

 pressed surface. The Poitevin process has been much improved ;both 

 by himself and by other inventors and operators, and M. Poitevin has 

 applied it to lithography, and to typographic or copper-plate repro- 

 ductions. To obtain a photographic image on stone, he operates as 

 follows : 



Some albumen or bichromate of potash is deposited on th"e grained 

 stone, which receives, when dry, a negative photographic proof; it is 

 then exposed to the light. The stone is found to be acted on, so that 

 the ink only adheres to the impressed parts, that is to those which 

 correspond to the shades and half tints of the image. The printing 

 off is continued, as for ordinary lithographic impressions. 



It will be apparent that it is the gelatine that receives the ink and 

 not the stone, and hence not many prints can be pulled off the stone. 

 To Sir Henry James, E.E., and Major de Courcy Scott, of the Ordnance 

 Survey, we are indebted for the first published method of practical 

 photolithography, though about the same time Osborne in Australia 

 brought out a somewhat similar process. Sir Henry James's plan 

 was to coat paper with gelatine, cover it with greasy ink, after exposure 

 beneath a negative of a zinc engraving or map, and float the back of 

 the paper on hot water. This caused the gelatine to dissolve and to 

 carry away with it the ink from those spaces which ought to be white. 

 After sponging carefully with a fine sponge to aid the operation, a 

 perfect facsimile of the original was presented to the eye. This was 

 then placed face down on a lithographic stone or on a zinc plate, and 

 after pressure in the lithographic press, the greasy ink left the paper 

 and adhered to the one or the other. Several other modifications 

 have been made of this method, but another, which is due to Captain 

 Abney, F.E.S., is perhaps an improvement. In his process a positive 

 picture is secured on gelatinized paper, the gelatine of which has 



