SUNLIGHT AND ITS MEASUREMENT 199 



The darkening of the silver halides, silver chloride and silver 

 bromide, upon exposure to light has been extensively inves- 

 tigated and the rate of darkening has been found to vary, 

 for one and the same light source, within certain limits, pro- 

 portionally to the product of the intensity of the active 

 radiation and the length of the time of exposure. This is the 

 so-called "reciprocity law." 49 Since, however, the degree of 

 darkening is difficult to determine, recourse is usually had to 

 Runsen and Roscoe's method of determining the time necessary 

 to produce a definite degree of darkening, determined by com- 

 parison with an unfading standard tint placed immediately 

 beside the darkening paper. From the reciprocity law it follows 

 that the intensity of the effective radiation is inversely pro- 

 portional to the time necessary for this fixed degree of darkening. 



This darkening is usually said to be accomplished by the 

 reduction of the silver salt to a sub-salt and the liberation of 

 the free halogen (CI or Br). If the light is removed the reaction 

 will reverse and the original salt will be formed. Since the 

 formation of the sub-salt is constantly opposed by the tendency 

 of the halogen to recombine with it, the speed of darkening de- 

 creases with increasing time. This may be avoided to a large 

 extent by adding another substance, such as gelatine or albumin, 

 with which the free halogen may combine, thus maintaining 

 the speed of reaction and, if the amount of this added substance 

 is sufficiently great in proportion to that of the silver, the decrease 

 in rate will be negligible. This reaction was first worked out 

 by Luther 50 and may be written: 

 2 AgCl + gelatine + light = Ag 2 Cl + chlorinated gelatine. 



Although the reciprocity law holds very exactly for a consider- 

 able range of intensity of monochromatic light (or light from 

 sources whose energy distribution curves are the same shape) 

 in the case of silver-chloride-gelatine preparations that are 

 viewed directly, it does not remain in its simple form in the 



49 Bunsen, R. W., and H. Roscoe, Photochemische Untersuchungen VI. 

 Meteorologische Lichtmessungen. Ann. Physik Chem. 117: 529-562. 1862. 



50 Luther, R., Studien liber umkehrbare photochemische Prozesse. Zeit. 

 Phys. Chem. 30: 628-680. 1899. 



