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HANDBOOK OF PHOTOGRAPHY 



Silver Halide Crystals. — The photographic properties of the emulsion are con- 

 trolled chiefly by the composition, size, and condition of the silver halide crystals. 



The slower positive emulsions are composed of silver chloride, silver bromide, or 

 mixtures of the two in various proportions. The faster negative emulsions consist of 

 pure silver bromide, or, more usually, silver bromide with a small proportion (3 to 

 5 per cent) of silver iodide. 



Fig. 1. — Grain structure of a medium-speed emulsion. (Magnification, about 1580 X. 



Fig. 2. — Grain structure of an ultrarapid emulsion, showing the hexagonal and triangular 

 shape of many of the grains. (Magnification, about 3500 X.) 



The grains vaiy in size from submicroscopic up to as much, in exceptional cases, 

 as 7 or 8 M in diameter. The available evidence indicates that all the grains, even 

 those below the limits of resolution of the microscope, are definitely crystalline. The 

 larger grains show the form of fiat hexagonal or triangular plates with rounded 

 corners, though occasionally needle-shaped crystals and irregular lumps appear^ 



■ Tbavelli, a. p. H., and S. E. Sheppard, "The Silver Bromide Grain of Photographic Emulsions," 

 Van Nostrand, (1921). 



