CHROMATISED GELATIN PROCESS. 377 



quently there is always more or less danger of the warm water 

 dissolving away, or at least softening the under less altered layers 

 of gelatin below the outer most altered ones, and so undermining, 

 as it were, the impression, and causing the film to be unduly 

 removed where the light action has been but comparatively feeble. 

 To obviate this it is usual to transfer the gelatin film from the 

 original paper to a specially prepared "transfer paper" by moisten- 

 ing the two with cold water and then carefully pressing them 

 together ; on allowing the whole to remain in slightly warm water 

 (about 37 C = 100 F.) the original paper (being more pervious to 

 water than the transfer paper) allows the solvent action of the 

 water to be exerted chiefly on the under surfac'e of the gelatin 

 film, and hence loosens its attachment to the original paper ; so 

 that by and bye the original paper can be carefully stripped off, 

 leaving the gelatin film adhering to the transfer paper. When 

 the development is complete and all soluble gelatin is washed 

 away, the film of pigmented gelatin is further indurated or 

 toughened by immersing in alum water (containing between 2 and 

 3 per cent, of alum) for a few minutes, and then washing with 

 plain water and drying. 



Instead of transfer paper, sheets of clear or opal glass, porcelain, 

 ivory, metallic plates, &c., may be employed, so that ultimately 

 the picture is formed upon them by transference of the gelatin film 

 from the paper employed to support it in the first instance ; all 

 such transfers, although positive as regards light and shade, are 

 reversed as regards right and left, just as an object seen by reflec- 

 tion in a mirror, or a transparent picture seen from the hinder 

 side. To avoid this, a " double transfer " process is used ; the 

 first transfer is made to the surface of a sheet of glass coated 

 firstly with wax (by pouring over it a solution of wax in benzine 

 which evaporates like the ether from collodion) and then with 

 ordinary (not iodised) collodion ; the coated glass is placed in water 

 to wash out the traces of alcohol and ether retained by the soft 

 fresh collodion film, and the paper supporting the exposed gelatin 

 film applied to it, so that the upper surface of the gelatin is next 

 the collodion ; on pressing the two closely together, and allowing 

 them to remain in slightly warm water, the transfer is made to 

 the collodion as above described. When the paper is stripped off 

 and the development complete, the film is now transferred a second 

 time from the collodion support to a sheet of prepared transfer 

 paper, somewhat hotter water being employed; so that the wax 

 film melts and allows the gelatin and collodion to leave the glass 

 plate and adhere to the paper. 



A good deal of dexterity and careful manipulation is requisite 



