IN THE current tests for distance discrimination based on the technic of Howard,1 the movable and the comparison rods are separated laterally by 6 cm. Howard did not give his reasons for selecting this value; it may have been with the intention of simplifying the geometric relations on which the calculation of the angle of binocular parallax is based. The average interocular distance is approximately 6 cm., and in Howard's figures the lines joining the eye and the rod on the same side are parallel.
The present experiment was designed to determine the effect on the threshold of the lateral separation of the targets. The centers of the test objects were separated laterally by distances of 2, 4, 8, 16 and 32 cm.; since the subject was seated 17 meters from the test objects, the angular values of the separations were, respectively, 4'3", 8'5", 16'11", 32'21" and
HIRSCH MJ, WEYMOUTH FW. DISTANCE DISCRIMINATIONII. Effect on Threshold of Lateral Separation of the Test Objects. Arch Ophthalmol. 1948;39(2):224-231. doi:10.1001/archopht.1948.00900020229009