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beginners in photography have observed the same thing when having 

 made two exposures of different objects on the same plate. In this 

 case it is easy to observe that not the sum of the two images is 

 obtained, but that in one place one object dominates, in another 

 place the other object. 



According to K. Schwarzschild the result in the case of the 

 exposure being intermittent depends, inter alia, upon the relation 

 between the interval and the duration of the separate exposures; 

 the longer the interval the better opportunity the halogen has of 

 escaping by diffusion, or of being absorbed by a chemical sensitizer, 

 and the more readily the next exposure will photochemically 

 decompose the « silver subhaloid germ, which is more sensitive to 

 light than the silver haloid, into /? silver subhaloid and halogen, 

 owing to which the result of development, apart from the photo- 

 chemical induction which is to be exceeded again, will remain below 

 the sum of the components. 



The difference in sensitiveness to light between silver haloid and 

 the a silver subhaloid appears, according to the above experiments, 

 to depend largely upon the kind of light with the seconday exposure. 

 The less sensitive the silver haloid and the more sensitive the a 

 silver subhaloid is to a given colour, the more pronounced Herschel's 

 effect will be. The smaller this difference, the more rapidly the 

 silver haloid will produce fresh a silver subhaloid germs; it is true, 

 in this case polarization is observable, but the minimum reducibleness 

 is soon reached. Further this is, of course, also dependent upon the 

 amount of « silver subhaloid, i. e. upon the duration of the primary 

 exposure. Perfect neutralization of reducibleness need not occur then. 



Consequently the colour sensitiveness occurring in the case of 

 Herschel's effect is to be ascribed to the colour sensitiveness of the 

 « silver subhaloid. Not one of the theories of the latent image 

 enunciated hitherto can account for the phenomenon in such a 

 simple way as the subhaloid theory. The subhaloids are dyes of 

 quite different colours from silver haloid, and consequently with 

 quite different spectra, owing to which the possibility exists of quite 

 different colour sensitiveness, as in fact actually appears from the 

 experiments of 0. Wiener 2 ). 



P. Villard proved spectroscopically that the greatest difference 

 between the liminal value of the silver haloid and the photochemical 

 induction of the re-silver subhaloid is situated in the red 



1 ) Probably these photochemical decompositions proceed according to an expo- 

 nential formula. 



2 ) J. M. Eder. Jahrb. f. Phot. u. Repr. 1896; S. 55, 



