\β’ nEN a photographic deposit is examined under the microscope, it is found to be made up of a large number of minute particles consisting of metallic silver which have been reduced from the silver halide by the action of the developer . It is necessary to employ a relatively high magnification in order to resolve clearly the individual particles or grains composing this silver deposit . The size of such grains varies over a considerable range, the homogeneity of grain size being dependent upon the particular type of emulsion, Measurements have shown that these grains vary in diameter fromapproximately 0 .5 microns in slow plates up to from t to to microns in high-speed plates . The distribution of these grains through the gelatine in which they are embedded is not absolutely uniform, there being in many cases a marked tendency for these grains to occur in groups or clumps of from five to twenty grains . Further, in some cases, these aggregates also tend to agglomerate, thus causing a further lack of homogeneity in the appearance of the deposit .
Considerable research work has been done by various investigators relative to the size of grain in the undeveloped plate as well as in the deposit resulting from the development of the exposed silver halide . Practically all of this work deals with the size of the ultimate silver halide crystals or the metallic silver particles, and with the relative proportion of grains of various sizes occurring in various emulsions . In some cases a developed deposit may appear grainy (that is, lacking homogeneity) to the unaided eye . The cause of this is almost invariably due to an agglomeration or grouping of the clumps made up of several of the ultimate silver particles which of themselves are so small as to be entirely beyond the resolving power of the unaided eye .