Fixation disparity was measured while the subject fixated a target 51.4 cm away in the median plane. Prism vergence was varied through a range of 20 prism diopters base-in to 20 prism diopters base-out. Subjective measurements were made with a modified haploscope. Photographic, or objective, measurements were made by placing contact lenses with plane mirrors on both eyes of the subject and reflecting light to an oscillographic camera, by using the optical-lever principle. The subjective and objective methods gave results which were in close enough agreement to support the conclusion that the mean primary uniocular visual direction does not vary as the fusional stress is changed. Photographic records made with steady binocular fixation of the target for over 1 min showed that the vergence of the eyes varies by less than 10 min of arc, which is consistent with the concept of Panum’s fusional areas.
FREDERICK W. HEBBARD, "Comparison of Subjective and Objective Measurements of Fixation Disparity," J. Opt. Soc. Am. 52, 706-711 (1962)