1878.] to tliG Beproduction of Maps and P/ans. 89 



breaking up and destroying the continuity of gradation. By the processes 

 o£ photocollotype, so called from the printing surface being of gelatine, these 

 defects are entirely obviated, and absolutely permanent photographic prints 

 may be produced in the printing press equal to silver prints in perfect 

 delineation of detail and delicate gradation of shade, but vastly superior 

 to them in permanence and cheapness of production. 



Poitevin was the first to recognise, so early as 1855, the fact that the 

 half-tones were better preserved on stones that had been treated with a 

 chromated colloid mixture if, after exposure to light under a negative, 

 instead of being inked all over and then washed with water to remove the 

 superfluous ink, they were first moistened and then inked in with a litho- 

 graphic roller charged with printing ink*. He seems, however, to have 

 always regarded the stone as the principal printing surface and treated it 

 by the ordinary methods of lithography. Only a few impressions could be 

 obtained from stones thus treated. 



In 1866, Messrs. Tessie du Mothay and Marechal, of Metz, discovered 

 that the stone or metal plate hitherto used as a printing surface might be 

 replaced by a mixture of isinglass, gelatine and gum, treated with an acid 

 chromate, and evenly spread upon a well polished metal surface ; because 

 if, after exposure to light under a photographic negative, such a gela- 

 tinous surface were moistened, greasy ink applied upon it with a roller 

 would adhere well to the parts of it that had been acted upon by light, and 

 would be taken up by those parts in proportionate quantities, according to 

 the intensity of the gradations of light and shade produced on them by 

 the action of light, and their consequent impermeability to water. Photo- 

 graphic prints in fatty ink reproducing the most delicate gradations of shade 

 without any apparent grain or break of continuity could thus be produced, f 



It will be seen that this process was based on exactly the same principle 

 as Poitevin's photolithography, but differed from it in the distinct recognition 

 of the colloid film as the printing surface. Messrs. Tessie de Mothay and 

 Marechal were also the first to recognise the necessity of adding a certain 

 proportion of acid or of oxydising or reducing agents to the chromate salt 

 used for sensitising the gelatine, with the object of rendering the colloid 

 surface more apt to receive the greasy ink and also of hardening the film so as 

 to enable it to withstand the wear and tear of printing. This they did by 

 exposing the sensitive plates to a high temperature before using, but the 

 effect was produced in great measure by the decomposition of the chromate 

 salts by the acids or other substances added to the colloid mixture. 



Messrs. Tessie du Mothay and Marechal printed off their ' phototype' 

 plates in a lithographic press in much the same way as ordinary lithographs, 



* ' Traite de V impression photographiqne sans scls d^argent^' p. 78. 

 t ' Thotographic News,' Vol. XI, p. 260. 



