204 Captain Abnoy on the Reversal of 



be treated with a soluble bromide of an alkali, such as of potas- 

 sium, and be again exposed to light in the camera, it will be 

 found that there is not such a rapid reversal of the image as 

 with the iodide, but that longer exposure is required to effect 

 it, the reason being that bromide of silver prepared with a 

 large excess of soluble bromide is still sensitive to light. If, 

 therefore, the light decomposes the soluble bromide on the 

 plate, liberating enough bromine to form fresh bromide of 

 silver with the subbromide formed by the preliminary expo- 

 sure, that freshly formed bromide, being sensitive to light, is 

 again reduced to the subbromide state by the same rays which 

 formed it. It will be evident, however, that reversal should 

 take place more rapidly with the soluble bromide present than 

 without it; and such is the case. 



It is useless to treat a silver bromide film with a soluble 

 iodide, since silver iodide is immediately formed, and the reac- 

 tions that take place are similar to those already described. 



If bromide of silver in collodion be exposed to the image in 

 the camera without the presence of any other substance, a 

 reversal takes place. Roughly speaking, the reversal takes 

 some sixty times more exposure to the light than is requisite 

 to produce the maximum ordinary effect. To trace the cause 

 of this reversal it is only necessary to treat the film with a 

 5-per-cent. solution of potassium nitrite, when it will be 

 found that the reversal does not take place. The same holds 

 true when the film is treated with any deoxidizing solution, or 

 if the plate be immersed in benzene or hydrogen. The cause, 

 then, of the reversal in this case is evidently an oxidation ; 

 and this may be further verified by treating the film, after a 

 preliminary exposure, with bichromate of potash, hydroxyl, 

 &c. ; it will then be found that the reversal takes place 

 much more rapidly than when these oxidizing agents were 

 absent. The same may be said of the mineral acids. 



If silver bromide be held in a gelatine film, the action of 

 light is somewhat different. If the plate be exposed in the 

 camera for a short time, say a few seconds, the image develops 

 in the usual manner and we have a negative image ; if it be 

 prolonged to, say, a minute, the image is reversed on develop- 

 ment ; a further exposure causes a negative image to be pro- 

 duced, whilst one much more prolonged causes a positive 

 image again to be formed on development. Here are four 

 distinct phenomena* which need explanation. To solve the 

 problem offered, plates should be exposed when saturated with 

 a solution of potassium nitrite as before, when it will be found 



* Mr. C. Bennett described these phenomena in the 'British Journal of 

 Photography ' in 187& 



