( 24) 



silver iodide collodion plate with silver nitrate, which in the pre- 

 sence of light is a powerful oxidizer '), and the question is whether 

 the so-called neutralization of solarization by silver nitrate is not to 

 be ascribed to this as well, and consequently is real neutralization. 



If silver haloids are allowed to be photo-chemically decomposed, 

 the great influence of the size of the grain at once becomes evident. 

 While fine grain silver chloride or bromide is decomposed rapidly, 

 the latter even more rapidly than the former, the directly visible 

 decomposition in the case of course-grain silver haloids is slower. 

 This can at once be accounted for by H. Luggin's modified rule. 

 At the surface of the silver haloid grain the liberated halogen can 

 escape more easily, or enter into composition; inside the grain it 

 acts regressively, so that the progression will decrease from the 

 surface to the centre. 



H. Luggin 3 ), too, refers to the same thing in the case of silver 

 iodide. But even if it is in a vei'y finely divided condition, the directly 

 visible photo-chemical decomposition does not take place rapidly. 

 Owing to its greater atomic weight the liberated iodine not only 

 diffuses more slowly, but moreover it is a solid subslance. By 

 absorption of this iodine, e. g. by silver nitrate, the directly visible 

 photo-chemical decomposition at once becomes more rapid, so that 

 it is clear why a silver haloid which is more sensitive may all the 

 same yield a less advanced photo-chemical decomposition. 



The surface decomposition of the silver haloid grain at the same 

 time points to the fact that here, too, the seat of the latent image 

 is to be looked for. This is also to be inferred from further data. 

 Thus the deposits of reduced silver haloids discovered by W. Schef- 

 PER s ) always start from the surface of the silver haloid grain, which 

 appears from a microphotograph published by him. Further Lüppo- 

 Chamek 4 ) pointed to the dependence of the quantity of dye in the 

 case of optical sensitization upon the surface to be coloured (i. e. 

 upon the size of the grain) with silver chloride and silver bromide. 



When it has been pointed out that in the appearance of solarization 

 by primary fixation and secondary development the existence of a 



!) M. Carey Lea (Phot, Korresp. 1887; S. 346) and Lüppo-Ceamer (Phot. 

 Korresp. 1907 ; S. 538) showed that silver subiodide is a substance which is 

 extremely easily oxidized. 



2) Zeitschr. f. phys. Chemie. 1897; Bd. 23; S. 611. 



3 ) Phot. Rundschau. 1907; Heft 6. 



*) J. M. Eder. Jahrb. f. Phot. u. Repr. 1902; S. 58. 

 , . , „ „ 1905; S. 61. 



