n :  one of a series of incomplete rings or circles used in some eye charts to determine visual discrimination or acuity
 
Landolt, Edmond (1846-1926), French ophthalmologist. Landolt and Louis de Wecker (1832-1906) published from 1880 to 1889 a multivolume treatise on ophthalmology that was once considered a definitive work on the subject. Landolt spent his career applying optics in the study of physiology. In addition to devising the Landolt ring, he described the histological formation on the outer nuclear layer of the retina.
 
 
 
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