( 19 ) 



The reappearance of reducibleness in the case of continued exposure, 

 the so-called second reversion of solarization might then again bo 

 attributed to a newly formed y-silver subhaloid, or, as the third 

 reversion has not been observed, to a metallic silver germ or what 

 is also possible, to both. 



It must be emphasized at the very outset, that it is by no means 

 impossible that before the «-silver subhaloid one or more other sub- 

 haloids, richer in halogen, are formed, which possess no germinative 

 property, for a primary exposure below the liininal value of the 

 plate points to photochemical decompositions taking place through 

 the occurrence of auto-sensitizing 1 ), so that the liminal value cannot 

 be considered identical with the photochemical induction. Nor must 

 it be inferred from the above that the «- or the ^-silver subhaloid 

 does not consist of more than one silver subhaloid. 



Consequently it appears from all this that sensitiveness to light 

 and reducibleness must on no account be identified, as is generally 

 done. 



The untenableness of the existing theories of Herschei/s effect by 

 Claudet, P. Villard, R. W. Wood and Warnerke is accounted for 

 by this faulty identification of reducibleness with sensitiveness. 



For the appearance of Herschei/s effect it is necessary that the 

 primary exposure should exceed the liminal value of the plate. Con- 

 sequently «-silver subhaloid must have been formed. 



That by the secondary exposure a regressive reaction occurs 

 between the «-silver subhaloid and halogen, cannot be assumed, 

 because in the case of prolonged exposures Herschel's effect coalesces 

 with solarization, in connection with which the formation of the 

 ^-silver subhaloid without germinative property has already been 

 stated. The experiments of W. Abney mentioned above, also prove 

 that halogen absorption promotes the phenomenon. Consequently 

 the secondary exposure acts in such a way that the «-silver sub- 

 haloid formed photochemically by the first exposure is reduced to 

 /J-silver subhaloid more rapidly than it has been possible for an 

 equal quantity of «-silver subhaloid to be formed afresh. (In this 

 case it may happen that the silver subhaloid has already entirely 

 been photochemically dissolved, before the silver haloid has been 

 able to supply it). This appears from the discussion of fig. 6. The 

 photochemical induction of the «-silver subhaloid is, therefore, lower 



] ) Vide: J. M. Eder. System der Sensitometrie phot. Platten. Sitzungsber. d. 

 kais. Akad. d. Wiss. in Wien 18Ü9; Ha; Bd. 108; S. 1407. J. M. Eder u. E 

 Valenta. Beitrage z. Photochemie. 1904; Bd. II; S. 48. 



2* 



