of the Photographic Image. 49 



quantities. A plate prepared with silver iodide was flowed 

 over with tannin and dilute albumen, and dried. It was then 

 exposed in the camera, and after exposure half of it coated with 

 an emulsion formed in collodion by silver bromide. Now a pho- 

 tographic image impressed on silver iodide is not amenable to 

 alkaline development unless the solutions be excessively strong. 

 An alkaline developer made as below was therefore employed : — 



1. Pyrogallic acid . . 16 grains. 

 Water 1 oz. 



2. Potassium bromide . 20 grains. 

 Water 1 oz. 



3. Liquor ammonias ('880) •§■ oz. 

 Water 8 oz. 



or Potassium hydroxide. 15 grains. 



Water ..... 1 oz. 



One part of Nos. 1 and 3 were added to every three parts of 

 No. 2. As might have been expected, but a trace of an image 

 was seen on applying the developing solutions to the uncoated 

 iodized plate (and this trace was subsequently proved to be 

 due to the sensitive albumen salt) ; but where the emulsion 

 had been used, an image gradually appeared, not very strong, 

 but still perfectly visible and of printing density. 



Silver bromiodide plates were treated in the same way; in 

 this case there was a feeble image on the part uncoated with 

 bromide emulsion ; but on that part coated with emulsion the 

 image appeared on the bottom surface of the emulsion film, 

 and gradually worked its way up till reduced silver was obtained 

 on the top surface, where the light had most strongly acted 

 on the exposed film. The recoated half-plate, on fixing with 

 potassium cyanide, gave a perfectly bright image, clear and 

 dense, whilst on the other half it remained feeble. 



With bromide plates in which only a feeble image could 

 be obtained, the same procedure gave the requisite density; 

 and this fact is likely to be of practical value. 



It was next proposed to ascertain in which film the de- 

 veloped image was really situated — whether in the exposed or 

 the unexposed film. The double films, which had been treated 

 as described, were taken from the glass plate by applying a 

 damped piece of gelatinized paper to the surface ; and after 

 detachment, a similar piece of paper was applied to the surface 

 which had been next the glass. When very nearly dry, the 

 two pieces of paper were pulled apart ; one film was found 

 attached to one and the other to the other. Considerable 

 difficulty was found in this operation, and only about 20 per 

 Phil, Mag. S. 5. Vol. 3. No. 15. Jan. 1877. E 



