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The Ophthalmetron, an instrument based upon the Foucault knife edge test for refraction, was used to measure refractive state of 100 patients who were also tested by subjective test and by streak retinoscopy. The agreement between Ophthalmetron and subjective test was as close as that between retinoscopy and subjective test.
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Knoll et al. (1970) studied this question.
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