208 On the Reversal of the Developed Photographic Image. 



chief oxidizing action lies in the red and ultra red of the spec- 

 trum. 



Fig. 11 also shows the effect of bichromate of potash on silver 

 bromide given a preliminary exposure, the plate in this case 

 being a gelatine plate. It will be seen that the bichromate 

 totally arrests all action in the blue, whilst it rapidly causes a 

 reversal in the red. 



Fig. 12 shows the effect of mineral acids on silver bromide, 

 by which it will be seen that a maximum of reversal takes 

 place in the red and in the blue. As before stated in regard 

 to the iodide, the action of these acids can scarcely be regarded 

 as an action of oxidation. 



Fig. 13 shows the phenomena due to overexposure of silver 

 bromide, by which it will be seen that reversal takes place in 

 the blue and not in the red. Comparing this with figs. 7, 11, 

 and 12, the effect of extraneous matter in causing a reversal is 

 very marked. 



Collodion plates exposed in benzene, or in aqueous solutions 

 of pyrogallic acid, potassium nitrite, and sodium sulphite gave 

 no reversal whatever. 



Gelatine plates exposed in benzene gave the phenomena 

 shown in fig. 13, whilst with the other media no reversal at all 

 was obtained. 



The explanation of the apparent contradiction shown by the 

 behaviour of a gelatine plate exposed in benzene has already 

 been given. 



The actions of many other liquids and gases* have likewise 

 been tried ; but it was thought that the examples given suf- 

 ficed, since they all pointed to the same conclusions, which 

 may be summarized as follows : — 



1st. The reversal of an image is due, in the majority of cases, 

 to the oxidation of the subsalt of silver which formed by the 

 first impact of light on the exposed salt of silver. 



2nd. The oxidation is due to the action of light, the rays of 

 lower refrangibility being the most powerful accelerators of 

 oxidation. 



3rd. Reversal of an image may be due to the presence of 

 any haloid of an alkali, the reversal in this case being partly 

 due to the action of light on such a haloid, and partly due to 

 the tendency to oxidation of the subsalt of silver. 



4th. The presence of a mineral acid tends powerfully to 

 cause a reversal. 



* Ozone was most marked in its oxidizing properties, and gave a curve 

 very similar to fig. 12, both with the iodide and bromide of silver, 



