Verhoeff's circles

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Verhoeff's circles 

Two black concentric circles designed for use with the duochrome test and as a target for the cross-cylinder method. The thickness and overall diameter of the inner ring are equivalent to a 6/6 (20/20) Snellen letter while the thickness and overall diameter of the outer ring are equivalent to a 6/15 (20/50) Snellen letter. Syn. Verhoeff's rings. See Snellen chart; cross-cylinder test for astigmatism; duochrome test.
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