( 20) 



than (lie liminal value of the silver haloid. Consequently the a-siher 

 sub halo id is a substance of greater sensitiveness to light than the 

 silver haloid. 



If with the secondary exposure the amount of « silver subhaloid 

 originally present is exceeded, a normal copy is obtained again. The 

 critical exposure, therefore, is that secondary exposure by which the 

 same amount of « silver subhaloid is formed as was present after 

 the primary exposure. So the best gradation of the polarized copy 

 in the case of Herschel's effect is obtained, if lower light intensities 

 are employed, as is shown by experiments. 



The amended proposition of H. Luggin states that with a certain 

 definite light intensity the progressive and the regressive reaction 

 in the silver haloid arrive at a state of equilibrium, if the liberated 

 halogen is not removed. This removal of halogen, either by diffusion 

 or by chemical sensitizers, is therefore of paramount influence upon 

 the origination of Herschel's effect. Consequently the most successful 

 experiment is obtained with an emulsion which immediately absorbs 

 the liberated halogen, or what is better even, if between the primary 

 and the secondary exposure the plate is put aside for a considerable 

 time, by which the liberated halogen is diffused out of the emulsion. 

 It is still simpler to treat the plate after the primary exposure with 

 a halogen absorbent, as was done by W. Abney, and we therefore 

 regard judson blue, mentioned by H. W. Vogel 1 ) as a substance 

 probably behaving analogously. 



Therefore the direct decomposition of the silver haloid by reducers 

 as in the case of J. Serry's experiments and fig. 5 and 6, in which 

 the liberated halogen enters into combination, acts so favourably 

 upon Herschel's effect. 



This makes P. Yillard's statement clear that not all emulsions 

 are equally suitable for experiment, as in the various emulsions 

 there are different chemical sensitizers (both in quality and in quantity). 



At the same time the nature of the phenomena in the case of 

 intermittent exposure becomes clearer now. 



That the effect of development upon silver bromide gelatine (but 

 not necessarily the photochemical decomposition) is always slightly 

 less than with a continuous exposure of the same duration was 

 observed by W. Abney 5 ), K. Schwarzschild 3 j, and others. Many 



i) H. W. Vogel. Handb. d. Phot. 1890; Bd. I; S. 221. 

 2 ) Photography 1893; p. 682. 



Phot. Archiv. 1893- S. 339. 



J. M. Eder. Jahrb. f. Phot. u. Repr. 1894 ; S. 373. 

 *) Phot. Korresp. 1899; S. 171. 



