On the Precise Objective Determination of Eye Movements

In the usual course of a prism-vergence test, the eyes are slowly forced to converge (or diverge) until the subject reports that he sees the test object double. Then the prismatic power is slowly decreased, and, at a certain point, fusion of the double images is suddenly regained. Just at this moment when recovery of fusion takes place, the two double images must have attained a certain critical separation, which is capable of initiating fusional movements of the eyes to overcome the diplopia. If, just at the moment of recovery, one of the double images falls in the fovea of one eye, the other image will necessarily fall extrafoveally on the retina of the other eye at a certain angle, an angle of disparity, which is one dimension of the so-called perimacular fusional area. 1 It has been generally assumed that in the prism-vergence tests, as soon as the images