PHOTOGRAPHIC MATERIALS 119 



(see Figs. 1, 2). Owing to the unidirectional shrinkage of the emulsion in drying, 

 these flat crystals are oriented parallel to the plane of the emulsion. ^ Though the 

 smaller grains appear only as round spots in photomicrographs, owing to limitations 

 of the resolving power, it is reasonable to believe that they belong to the same crystal 

 species as the larger grains. 



In any one emulsion the large grains show, as a class, higher sensitivity than the 

 smaller grains, though this difference is statistical and there are many individual 

 exceptions. 



In bromoiodide emulsions the large grains also contain a higher proportion of 

 iodide; this may be a contributing factor in the greater sensitivity, but it is not 

 the only factor since the grains in pure bromide emulsions show a similar increase in 

 sensitivity with size.^ 



The sensitivity to light, or rather to developability, is not uniform over the surface 

 of the grain but is concentrated at one or more sensitivity specks. It has been estab- 

 lished that these sensitivity specks are caused by minute quantities of silver sulphide 

 in the crystals. These impurities probably produce strains in the crystal lattice and 

 tend to concentrate or orient the products of light action from the adjacent portions 

 of the crystal in order to form developable nucleii. 



Since the grains of different sizes differ in sensitivity, the characteristics of the 

 emulsion depend to a considerable extent on the way in which its grains are distributed 

 along the various sizes. The size relations of the grains are best shown by size- 

 frequency curves in which the number of grains falling in each class size, expressed as 

 a fraction of the total number of grains, is plotted against the class size. In general, 

 emulsions containing very small grains are slow, and the sensitivity increases as the 

 average grain size is increased. When the majority of the grains are in one size group, 

 i.e., when the size-frequency curve shows a high maximum with only low wings, the 

 emulsion tends to be more contrasty; if the sizes are more widely distributed, the 

 maximum contrast tends to be lower. These relations are only general and are modi- 

 fied by other factors affecting the grain sensitivity, such as the extent of formation 

 and the distribution of sensitivity specks. The size-frequency curves for the grains 

 in two typical emulsions are shown in Fig. 3. Curve B is for a slow lantern-slide 

 emulsion that has comparatively low speed and high contrast; curve A is for a portrait- 

 film emulsion which has comparatively high speed and low contrast. 



The size distribution and the sensitivities of the grains are determined by the 

 conditions of precipitation and ripening of the emulsion. The initial grain sizes are 

 determined by such factors as the concentrations of the silver nitrate and potassium 

 bromide solutions, the quantity of gelatin present, and the rate of mixing. The final 

 size distribution is affected by the nature of the original precipitate and by the con- 

 ditions of digestion. The presence of silver halide solvents such as excess potassium 

 bromide or, in the case of alkaline digestion, of ammonia, seems to favor the production 

 of larger grains through Ostwald ripening, by facilitating the solution of the smallest 

 grains. The sensitivity specks of silver sulphide are also formed during the digestion, 

 and their production is controlled by the conditions of the digestion and the nature of 

 the gelatin.^ 



Other Emulsion Components. — In commercial practice, various substances in 

 addition to the gelatin and silver halide are added to control the properties of the 

 emulsion. Sensitizers to increase the light sensitivity may be either "chemical." 



1 SiLBERSTEiN, L., The Orientation of Grains in a Dried Photographic Emulsion, /. Optical Soc. Am., 

 5, 171 (1921). 



2 Sheppard, S. E., and A. P. H. Travelli, The Sensitivity of Silver Halide Grains in Relation to 

 Size, J. Franklin Inst., 203, 829 (1927). 



3 Chibisoff, K. W., "Ripening of Photographic Emulsions," p. 405, IXth Cong, intern, phot., 

 Paris (1935). 



