50 Captain Abney on the Alkaline Development 



cent, of the whole were properly manipulated, through our not 

 being always able to hit on the exact amount of desiccation. 



In examining these films differences were observable. In 

 some cases the image was found to lie almost entirely in the 

 exposed film, whilst in others a strong image was obtained in 

 the unexposed film. The difference was eventually found to 

 depend on the alkali used in the developer, and on the porosity 

 of the collodion in which the emulsion was formed. When 

 ammonium hydrate was employed and a porous emulsion, 

 the image was On the exposed film ; whilst if potassium hy- 

 droxide were employed, a vigorous image was in the top un- 

 exposed film. This can be explained through the solubility 

 of silver bromide in ammonium hydrate. The silver bromide 

 would be first dissolved by the ammonium, and then during 

 the course of reduction be carried down to the reduced silver 

 beneath it. Since silver bromide is insoluble in potassium 

 hydroxide, the haloid would be reduced in situ. 



Having determined this point, it was next endeavoured to 

 ascertain if the photographic image impressed by light on the 

 bottom surface caused a sympathetic action in the unexposed 

 emulsion film. Plates were prepared as before, one half being 

 coated with emulsion after exposure, and put aside for some 

 days. The films were then separated as described, and to 

 each the developer was applied separately. With one or two 

 exceptions, no image teas obtained on the unexposed film. The 

 reason of an image (always imperfect) being obtained in some 

 cases was traced to the adhesion to it of the thin layer of 

 albumen, with which the exposed film w T as coated ; for when 

 the albumen was applied after exposure, no image was de- 

 veloped. 



The foregoing experiments clearly show that the photo- 

 graphic image has no power of transferring itself, or of 

 creating a sympathetic action in an unexposed film previous 

 to development, and that therefore the increase of density 

 and formation of a secondary image must be due to other 

 action than chemical. 



If further proof be required, it is only necessary to expose 

 a dry plate, and develop it in the ordinary manner, and after 

 drying to coat it with an emulsion, and develop again. It will 

 be found that where the metallic silver of the first' image 

 is beneath it, there the top film develops and gives a coun- 

 terpart of it. The metal exercises an attraction for the 

 silver on the point of being reduced from the bromide in 

 a similar way that it does when a silver tree is built up, or 

 when a developed image is intensified by the application of 

 pyrogallic acid and silver nitrate. There seems to be a 



