(3) 



plastic shape similar to that formed by the pigment-gelatine printing 

 process, the insoluble part being a normal copy, the soluble part a 

 polarized one (monochrome pinatypy). 



It is characteristic of the normal copy, if exposed and developed 

 normally that the portions where the density is greatest show a 

 duller surface when seen by light reflected at an angle than the 

 clearest portions. This is to be attributed to the presence of reduced 

 silver-haloid grains immediately under the free surface. 



If the image lies against the free surface, it may be called a 

 surface image, in opposition to a "depth" image, with which this is not 

 the case. 



The cause of the appearance of the surface image has been ascribed 

 by P. H. Eykman and myself 1 ) to the surface tension of the wet 

 gelatine. So a silver-haloid gelatine layer may be looked upon as 

 consisting of a series of layers, of which the top one, i.e. the one 

 at the free surface, is the most sensitive, while every succeeding 

 one lying under it, is less sensitive. The exposure required to render 

 a beginning of reduction by the developer visible, the liminal value 

 ("Sch wellen wert") consequently seems to increase in proportion to 

 the depth; that of the topmost layer is equal to the liminal value 

 of the plate itself. We will call this the "absolute", that of the 

 succeeding layers the "relative" liminal value. 



Leaving aside, for clearnes's sake, the mutual differences in 

 sensitiveness of silver-haloid grains in one single layer, to which 

 J. M. Eder 2 ) and J. Plener 8 ) have drawn attention, the differences 

 in size and shape and the topographic situation of the grains, the 

 normal copy may be represented as is shown in cross section and 

 graphically in fig. 1. 



The shape of the image of a polarized copy might be represented 

 as indicated in fig. 2. I have found that this explanation cannot be 

 applied to a single photographic image, but it is applicable to chemi- 

 graphic processes, catatypy and the silver-pigmentgum process. 



In the case of some polarized copies, as the counter-positive and 

 -negative, a normal copy is developed first, the reduced silver haloid 

 of which is dissolved and, after a diffused exposure, redeveloped. 

 Now a depth image originates (fig. 3). Owing to the diffused exposure 

 the base of it, leaving aside slight differences in light absorption, 

 will everywhere be about equally distant from the free surface. 



i) Drude. Annal. d. Phys. 1907; Bd. 22; S. 119. 



2) J. M. Eder, Handb. d. Phot. 1902; Bd. Ill; S. 64. 



Phot. News. 1883; p. 81. 

 ») Phot. Korresp. 1882; S. 306. 



1* 



