The<unk>  is a black disk with a thin<unk>  through it used by optometrists in detecting the level of an astigmatism. The disk is rotated so that the<unk>  is oriented at different angles, changing the retinal blur size. The<unk>  is more useful to screen for a high degree of astigmatism in patients who have poor vision.
Stenopaeic slit
Optic cup 
Clitoral hood
Ranula
Cortical homunculus
Corneal button
Limbal ring
Munk's Roll