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Edmund and Clemmesen have devised an accurate clinical test for determining and recording the degree of visual disturbance present in persons who are suffering from a deficiency in the adaptation of the eyes to reduced illumination. Years ago Edmund used for these tests a white Masson disk on which were drawn eight black sectors of different sizes. When the disk was rotated there appeared eight gray rings of different shades ranging from a nearly white tone to one almost black. The power of distinction, as he calls it, could then be determined by finding the palest gray ring which the observer could distinguish from the white ground. For clinical use this test was modified by the authors, and large test letters of different gray tones on a white ground were substituted for the gray rings. These letters were viewed through closely fitting test frames, excluding side light, in which
Holden WA. On Deficiency of A Vitamin and Visual Dysaptation. Arch Ophthalmol. 1936;16(6):1072–1073. doi:10.1001/archopht.1936.00840240172021
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