LIGHT AND VEGETATION 125 



Thus, in spite of the simpHcity of the photographic action 

 compared with the photosynthetic action, new analogies 

 appear and the theory of Gurney and Mott provides a pointer 

 towards the solution of the difficulties mentioned at the end 

 of the last chapter. 



What constitutes the photographic action of light on 

 silver bromide? If bromide paper, i.e., paper covered with a 

 layer of gelatine in which a number of grains, or small crystals, 

 of silver bromide are submerged, is exposed to sunlight, it 

 soon turns black. Each grain of bromide is progressively 

 decomposed; the bromine escapes and leaves the silver in the. 

 form of colloidal black grains. 



If the sensitized paper, instead of being exposed to intense 

 sunHght, is partly exposed to a lower illumination for a few 

 seconds only, no blackening is visible. Nevertheless a modi- 

 fication has taken place; a "latent image" has been formed, 

 for when the paper is plunged into a suitable reducing solution 

 (the developer) the parts which were exposed to the light 

 are seen to darken in a few minutes, while the parts which 

 were protected from the light still remain white. This is called 

 the development of the latent image, which prolongs the invis- 

 ible action of a short exposure to Hght. It is agreed today that 

 this latent image is imprinted in the gelatine emulsion in the 

 form of a few atoms of free silver in the grains of bromide 

 on which the light has acted. 



One can explain why the developer attacks these grains 

 in preference to others. Thus, whether the light is made to 

 act for a short period to give an invisible latent image which 

 can be brought out in a developer, or for a longer period until 

 . blackening is produced, the photochemical effect is manifested 

 by the liberation of metallic silver. 



Two observations play an important part in the theory. 

 The first is that a large crystal of silver bromide — an electrical 

 insulator — becomes a conductor while it is receiving the 

 light; the second is the accumulation of silver, in small 

 crystals of irradiated bromide, in grains visible under the 

 microscope. Now, the quanta of Hght are absorbed in each 



