As White has frequently pointed out, the determination of the type and the degree of heterophoria in the primary position, for distance and for near vision, is meaningless from the diagnostic standpoint unless the behavior of the individual ocular muscles is studied in the different cardinal positions of gaze : up and right, right, down and right, up and left, left and down and left. It is only through this examining procedure that paralyses of the extraocular muscles can be accurately diagnosed and the motor anomalies they produce clearly differentiated from purely innervational disturbances of the convergence and divergence centers.
Unfortunately, in daily practice the routine investigation of the extraocular muscles is frequently done in the primary position only, for distance and near, with the Maddox rod. Owing to pressure of time or lack of familiarity with the screen test and prism measurements, the diagnostic positions of gaze are studied only