Edmond Landolt1846–1926

Landolt Cs

Snellen’s isolated shapes were called optotypes, and a modification of them, avoiding rectangular elements, was introduced by Landolt in 1888. The Landolt C or ring consisted of a circle with a gap in it; the size and orientation of the gap could be varied, and the observer’s task was to detect the orientation of the gap in the ring. Landolt was born in Switzerland and worked with Snellen in Utrecht before becoming professor of ophthalmology at the Sorbonne. Together they published an influential analysis of measurements in ophthalmology. After some competition between the Snellen’s E and Landolt’s C, the latter became the favoured stimulus for laboratory studies, and Snellen’s letter chart was adopted by clinicians. It has been said that the history of visual acuity is littered with still-born charts. Those of Snellen and Landolt continue to display rude health. The portrait of Edmond Landolt is combined with ‘acuity’ written in circular letters, the C of which has its opening right, above, left, and below.