of the Photographic Image. 51 



further action, however, which must be taken into account. 

 It has already been shown that silver subbromide is more 

 easily reduced to the metallic state than is the bromide. In 

 these last experiments I have found that there is great dif- 

 ficulty in starting the development in the unexposed bromide 

 film if the layer of albumen be too thick, but that when once 

 started it proceeds more rapidly. This can be accounted for 

 on the supposition that the attracting particles of silver were 

 too far distant from the emulsion film to exercise their attrac- 

 tive power. 



But it seems, from other experiments which are still in pro- 

 gress, that the atom of reduced silver immediately combines 

 with the nearest molecule of silver bromide and forms subbro- 

 mide, which is reduced to the metallic state, and its two atoms 

 of silver combine w T ith two other molecules of silver bromide, 

 and so on, the image being gradually built up in this manner. 

 Workers accustomed to alkaline development must have no- 

 ticed that the feeble image first formed in the film grows in 

 intensity rapidly up to a certain point and then flags: the 

 reason of this is apparent if we consider the above reaction to 

 take place, and the subsequent exhaustion of available silver 

 bromide which can be acted upon. 



One more experiment must be noted. If the film of albumen 

 or gum &c. between the two films be very thick, a reversed 

 action will take place, which can be explained by the fact that 

 the strength of the developing agent is exhausted in producing 

 the image on the exposed parts of the under film, whilst on the 

 other portions the silver bromide most readily attackable is in 

 the upper film. 



I have already indicated that this application of a film of 

 silver bromide after exposure, either before or after development, 

 might prove useful when employing plates which only yield 

 a thin image when developed in the usual manner: other 

 applications will suggest themselves. I have also previously 

 pointed out* that intensity may be given to an image by- 

 alkaline development if a silver compound soluble in the alkali 

 be gradually added to the developer. This new method, how- 

 ever, seems the preferable one to adopt. 



* Photographic News, March 27, 1874 



E 2 



