(25) 



silver subhaloid can be proved, this is not enough to account for 

 the solarization phenomenon. 



In order to illustrate this let us assume a photo-chemical decom- 

 position with direct recomposition of the liberated halogen. 



If the exposure of a photographic plate is prolonged, the silver 

 haloid will keep forming a silver subhaloid, which is the germ for 

 development. From Herschel's effect, however, it appears that this 

 a silver subhaloid in itself is a highly light-sensitive compound, so 

 that it is not to be assumed that a continual accumulation of germs 

 is taking place. Consequently the a silver subhaloid rapidly decom- 

 poses into /J silver subhaloid and halogen. At the surface of the 

 silver haloid grains a condition therefore arises in which the number 

 of germs present depends upon formation and destruction. 



If the quantity of the remaining grain surface silver subhaloid 

 decreases, the quantitative formation of a silver subhaloid will also 

 decrease, and as the latter itself is highly sensitive to light, the 

 consequence of this will also be a quantitative decrease of the number 

 of remaining germs, in other words, the reducibleness will decrease, 

 i. e. solarization will set in. 



This phenomenon is, therefore, entirely dependent upon the avail- 

 able surface of the grain (size of the grain). In his experimental 

 researches Lüppo-Cramer *) repeatedly pointed to this fact. 



In reality this, of course, does not take place so rapidly. The 

 various makes of plates have chemical sensitizers which differ from 

 each other (qualitatively and quantitatively). This, together with the 

 prevention of diffusion, is the reason why the different commercial 

 plates begin to get solarized after mutually different exposures. 



The thiosulphate reaction shows peculiarities which can be accounted 

 for now. The subhaloids are decomposed by the action of thiosul- 

 phate into silver and halogen silver, which after being converted 

 into silver thiosulphate, dissolves as a double salt. The place of the 

 a silver subhaloid germ and the ft silver subhaloid is consequently 

 taken by silver, which also possesses the property of germination, 

 as appears from the development of primarily fixed plates. Strong 

 solarization, however, still produces solarization during development 

 after primary fixation, so that the reaction between ft silver subhaloid 

 and the thiosulphate in the binding material is a slow one, as is the 

 oxidation process already referred to. So if a highly sensitive course 



!) Phot. Korresp. 1901 ; S. 350. 

 Lüppo-Gramer. Wissensch. Arbeiten. 1902; S. 41. 

 Lüppo-Crameb. Phot. Probleme 1907 ; S. 146. 



