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I have not infrequently encountered cases of astigmatism in which, after having corrected the error in each eye separately, and on testing both eyes simultaneously, in binocular vision, have found that vision proximum was not perfect, and in order to attain normal vision near at hand, in binocular sight, the angle denoting the axis of the cylindrical glass must be changed in one or both eyes.
In correcting this binocular defect, types and the astigmatic bars were employed. If the patient looks upon the floor it will seem to incline to the right or left, and on changing the axis of one or both cylinders, the surface will appear level. But the same defect will be apparent if a board 12 × 3 inches, with parallel sides, be held in front of the patient at one metre, and on a level with the eyes.
In these cases then, the answer
CULBERTSON H. BINOCULAR ASTIGMATISM.Read in the Section on Ophthalmology at the Thirty-ninth Annual Meeting of the American Medical Association, at Cincinnati, May 7, 1888.. JAMA. 1888;XI(18):622–625. doi:10.1001/jama.1888.02400700010001c