LIGHT AND VEGETATION 129 



the contrary, it increases in very low illuminations given for a 

 proportionately longer exposure time. 



In other words, if the same quantity of light is sent several 

 times on to different parts of the same sensitive plate, first by 

 strong illumination of short duration, then by lower and 

 lower illuminations of longer and longer duration, the 

 darkening of the plate will become fainter and fainter, at a 

 temperature maintained at +50'' C, and, in contrast, stronger 

 and stronger at a temperature of - 75° C. 



These observations may be compared with those relating 

 to photosynthesis. The latter, although more complex, may 

 be summarized, as regards the essential facts, as follows: at 

 ordinary temperature, strong illuminations of short duration 

 are more effective than continuous illumination of the same 

 average value. The influence of variations of temperature, 

 within the limited range in which the plant can still live 

 normally, is difficult to determine and depends on other 

 conditions. To consider only this factor, as in photography, it 

 assists or checks the action of hght according to the level of 

 illumination. 



It is therefore interesting to have, for the simple case of 

 photographic blackening, the explanation given by Gumey 

 and Mott of phenomena analogous to those of photosynthesis. 



In strong illumination, free electrons are abundantly 

 produced in the silver bromide; in its formation the grain 

 of silver collects a larger number, and its increase in size 

 depends only on the speed at which the Ag+ ions detached 

 from the lattice by thermal agitation can reach it. The higher 

 the temperature, the more numerous the ions are and the 

 more freely they move. At a low temperature they do not 

 arrive in sufficient quantity to neutralize the electrons being 

 Hberated by the light and the formation of the grain of silver 

 is checked. 



At very low illuminations, the stray Ag-f- ions are 

 numerous enough not to hamper the increase of the grain of 

 silver, but the photo-electrons are rare. Gurney and Mott 

 explain that the grain of silver in formation captures them 



