Medicine:Stenopaeic slit
From HandWiki
The stenopaeic slit is a black disk with a thin slit through it used by optometrists in detecting the level of an astigmatism.[1] The disk is rotated so that the slit is oriented at different angles, changing the retinal blur size. The stenopaeic slit is more useful to screen for a high degree of astigmatism in patients who have poor vision.
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