By N. M. S. Langlands. Price, 2 shillings, 6 pence, net. London : His Majesty's Stationery Office, 1929.
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The measure of the visual acuity is the minimum angle between two points where images are just capable of being separated, and it has the value of 1 minute. For a "vernier" consisting of two half lines connected by a line, an angle of 10 seconds between the verticals is sufficient for certain resolution. This is the measure of "vernier acuity."
"Binocular acuity" is the minimum resolvable difference in depth, and is measured by the smallest difference of binocular parallax, which can just be appreciated.
The first recorded measurements were made by von Helmholtz, who obtained an angle eta (η) of 60.5 seconds. An interocular distance of 68 mm. subtends this angle at 230 meters, which he considered his limit of stereoscopic vision. Stratton (1898) found a radius of stereoscopic vision of 580 meters, with an angle of 24 seconds. Later observers found still smaller angles. Howard (1919-1920)
Lynch JB. Medical Research Council. Report of the Committee on the Physiology of Vision : IV. Experiments on Binocular Vision. Arch Ophthalmol. 1930;4(1):157–158. doi:10.1001/archopht.1930.00810090167018
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