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A considerable portion of the principal essay in this brochure is devoted to a detailed and critical study of the method employed for the determination of the power of distinction and its use in the detection of visual dysaptation (nyctalohemeralopia). Using various Tscherning neutral filters and a series of Snellen letters (6/36) of different shades of gray, with Bouguer-Fechner fractions varying in geometric progression, these investigators determined the power of distinction (logarithm of the reciprocal value of the B-F fraction) at different levels of illumination for groups of hospital nurses, pregnant women, and male and female patients selected at random. Details are presented regarding the determination of correct B-F fractions represented by the gray letters used in order that powers of distinction may be indicated in absolute values, thereby making possible a comparison of the results of various investigators. A number of pages are devoted to the experimental technic, errors
On Deficiency of A Vitamin and Visual Dysaptation. Arch Intern Med (Chic). 1937;59(3):557–558. doi:10.1001/archinte.1937.00170190192010
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