the Developed Photographic Image. 203 



The above experiments show, then, that a reversal may be 

 obtained by the presence of the iodides or bromides (and in a 

 more feeble manner, I have also found, by that of the chlorides), 

 and also by oxidizing agents and mineral acids ; whilst the 

 presence of a deoxidizing agent, or the exposure of the plate 

 in a medium free from oxygen, prevents the occurrence of the 

 phenomenon. 



We shall consider shortly as to whether the reversing action 

 depends upon the sensitiveness of the salt of silver obtained 

 by the preliminary exposure, or upon that of the agents em- 

 ployed in effecting such reversal. 



tVith the bromide of silver we have rather different phases 

 of the phenomenon to consider. The development can be 

 carried out with the alkaline or the ferrous oxalate developer, 

 a mode which is more easy to carry out than the development 

 by precipitation of metallic silver from an aqueous solution of 

 silver nitrate. For experimental purposes, films containing 

 silver bromide may be formed of collodion or of gelatine ; 

 and the behaviour of the silver-salt in the two vehicles is 

 somewhat different, and has to be considered separately. Col- 

 lodion is, or should be, a strictly neutral substance ; that is, 

 it is merely a medium in the pores of which the silver-salt 

 is entangled and kept in position, and has no effect on the pro- 

 gress of development or on the action of light, beyond that 

 which may be due to its physical qualities, its chemical con- 

 stitution remaining unchanged. 



A collodion film is essentially porous and not continuous, 

 as may be seen by a microscopic examination ; and free access 

 of the atmosphere to the silver is thus obtained. Gelatine, on 

 the other hand, is a substance readily acted upon by oxidizing 

 agents and by the halogens ; and consequently it may have 

 an effect on the progress of development and on the action of 

 light, its chemical constitution becoming altered. It is a ho- 

 mogeneous film, and not porous in the ordinary sense of the 

 word, and is a protective agency against the atmosphere to 

 those silver-salts which may be embedded in it. 



The most convenient method of experimenting with silver 

 bromide is in the form of emulsion, made either with collodion 

 or with gelatine ; but it is not to the purpose of the present 

 paper to refer to the mode of preparation beyond stating that 

 in the former case the emulsion is usually prepared with an 

 excess of silver nitrate, and the latter with an excess of soluble 

 bromide, both of which are eliminated as far as possible by 

 washing. 



If a film containing silver bromide, whether in gelatine or 

 collodion, have a preliminary exposure given to it, and then 



