If the positions of the eyes are recorded photographically1-5 or observed with a telescope 6, 7 during the process of making a horizontal vergence movement, it can be demonstrated that the record of the relative position of the lines of sight of the two eyes may differ from the convergence angles of the test targets during single binocular fixation. The exact amount of this disparity differs according to the subject and the test conditions, but there is fairly good agreement that the order of magnitude is several degrees, i. e., much larger than the size usually postulated for Panum's fusional areas,8 On the basis of such evidence, Tani, Ogle, Weaver, and Martens5 have questioned the validity of optical measurements of eye movements by photographic or telescopic observation of details on the eyes themselves or of images from light reflected from the corneas. These authors believe the
ALPERN M. The Position of the Eyes During Prism Vergence. AMA Arch Ophthalmol. 1957;57(3):345–353. doi:10.1001/archopht.1957.00930050357005
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