LIGHT AND VEGETATION 127 



alteraately, so that a bromine ion Br— is surrounded with 

 6 silver ions Ag+ and, reciprocally, an ion Ag+ is surrounded 

 with 6 ions Br— . 



In a photographic emulsion, these grains have Unear 

 dimensions of the order of one micron ; like the grana of green 

 plants, they contain a number of ions amounting to hundreds 

 of thousands of milUons. In moderate illumination, they each 

 absorb about a hundred photons per second. 



We have said that they become conductors of electricity 

 under the influence of Hght. This is explained by assuming 

 that a bromine ion, having absorbed the energy of a photon, 

 ceases to retain its supplementary electron and becomes a 

 neutral atom, while the electron escapes in the crystal lattice. 

 An electron, in such a lattice, as is shown by calculation, 

 is free to move in almost any direction; it can therefore be 

 instrumental in carrying a current in an electric field. These 

 electrons, having become free and being set in motion by 

 thermal agitation, may obviously finish up by finding again 

 either the atom of bromine that they have left, or another 

 one, recombining with it and restoring the initial state. But, 

 if there is already in the bromide crystal a small aggregate of 

 silver, a consideration of the energy levels in the bromide 

 and in the silver shows that a free electron passing into the 

 vicinity of a grain of silver will be captured by it and will 

 be unable to escape. 



If, therefore, grains of silver are present in a bromide 

 crystal, they will accumulate to themselves nearly all the 

 electrons liberated by hght until the negative charge of 

 electricity acquired is sufficient to repel the oncoming 

 electrons unless the grains of silver can become discharged 

 by receiving positive charges. This is exactly what happens. 



At ordinary temperature, the crystal lattice does not 

 possess the perfect regularity described. As a result of thermal 

 agitation, a small number of Ag4- ions, positive ions, are not 

 in their place, but have become intercalated in the lattice ; for 

 example, in the cubes whose outsides have ions of the normal 

 structure there are "voids" at the centres. These ions are able 



