Methods of expressing the size of sedimentary particles are briefly reviewed, and the use of projection areas is chosen as a method of measurement adapted to the study of pebble orientation. A photocell apparatus is described in which pebbles are rotated in a parallel beam of light before the cell. The cell is attached to an ammeter which measures the relative magnitude of the shadow cast by the pebble. Three mutually perpendicular axes are measured, which may be used to express the size of any pebble, however irregular. The axes are defined operationally, to avoid subjective factors in their measurement. The apparatus is used in connection with a study of pebble orientation, to determine the statistical distribution of the several axes in space, as they occur in the original deposit. For this purpose a triaxial ellipsoid is fit to the axes, and the orientation of the ellipsoid is expressed in terms of its longest axis and the pole of its maximum projection plane. These two reference lines permit the complete orientation of the pebble in space. By a slight modification of the techniques described, the sphericity values (Wadell) of the pebbles may be obtained by the apparatus.

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