Snellen and his optotypes

: Herman Snellen (1834-1908) studied medicine in Utrecht and wrote a thesis under the guidance of Franciscus Donders (1818-1889). In 1858 he became assistant and a few years later first physician at Donders' newly-founded eye hospital. In 1862 he published his optotypes, letters of different sizes, constructed in squares with 5 x 5 subdivisions. For each row of letters of a given size there is an indication of the distance D at which a normal eye can just distinguish these (corresponding with 5 minutes of arc). If the eye of the subject being examined can just make out letters of a given size at distance d, the visual acuity V for that eye is d/D. Snellen was a gifted surgeon for a variety of ophthalmologic disorders. In 1877 he was appointed Professor of Ophthalmology at Utrecht and in 1884 he succeeded Donders as director of the eye hospital.