抄録
Stroboscopic motion between two triangles in a symmetrical relation was studied in terms of probability of seeing rotation in depth as affected by configurational properties of the triangles, the axis of symmetry and the position of a viewing point. Symmetry was found to be primarily important for seeing the rotation. Corresponding line-elements of the triangles paralleling side by side near to the axis were particularly effective. Neither the orientation of the axis (vertical, horizontal of diagonal) nor the position of a viewing point (on or out of the axis) were effective.
