An analysis of grain distributions around a radioactive line source (consisting of polystyrene-3H) showed that the shape of the distribution was independent of the factors that influence resolution, i.e. section and emulsion thickness, silver halide crystal, and developed grain size. These factors did effect the spread of the distribution, however, and thus the distance from the line source within which 50% of the total developed grains fell. We called this distance "half distance" (HD) and determined it for a variety of specimens. When grain distributions were normalized in units of HD, one could plot universal grain distributions for specimens with radioactive sources of various shapes. The use of HD and the universal curves in interpreting radioautograms is discussed.

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