- Snellen's test types
Chart imprinted with lines of black letters graduating in size from smallest on the bottom to largest on top. Used for testing visual acuity. To have normal eyesight, one should be able to read with either of one's eyes, the line on the Snellen Chart marked 20 feet.
Test types were invented in 1843 by Heinrich Kuechler (1811-1873) in Darmstadt, Germany, and were improved by the Vienna oculist Eduard Jaeger Ritter von Jaxtthal (1818-1884) in 1854. Shortly after this Herman Snellen of Utrecht invented his chart of square shaped letters. This chart soon gained acceptance in all civilized countries.
- H. Kuechler:
Schriftnummerprobe für Gesichtsleidende. Darmstadt, 1843.
- E. Jaeger:
Schriftskalen. 3rd edition. Wien,, L. W. Seidel, 1860.
Jaeger first introduced his test types in 1854.
- H. Snellen:
Probebuchstaben zur Betimmung der Sehschärfe. Utrecht, P. W. van de Weijer, 1862.