Marius Tscherning

Marius Hans Erik Tscherning (December 11, 1854 - 1939) was a Danish ophthalmologist. He studied ophthalmology under Edmund Hansen Grut (1831-1907) in Copenhagen, and afterwards became a co-director of the ophthalmological laboratory at the Sorbonne in Paris. Here he worked with Louis Émile Javal (1839-1907). Tscherning spent 25 years at the Sorbonne, where he attained directorship of the ophthalmological laboratory. In 1910 he returned to Denmark as a professor at the University of Copenhagen and head of the ophthalmic department at the Rigshospitalet.

Tscherning is remembered for contributions made in optical physiology. He conducted research regarding entoptic phenomenon, Purkinje images, the etiology of myopia, and Listing's law of ocular movement. He also designed an ophthalmophacometer, which is a device used to measure changes that happen in the front and back curvatures of the lens during accommodation.

He is probably best known for his theory regarding the mechanism of accommodation, in which he disagreed with the accommodation theory proposed by Hermann von Helmholtz (1821-1894). Tscherning believed that accommodation occurred through an increase of zonular pressure at the lens equator with contaction of the ciliary muscle, and therefore a bulging of the lens in accommodation was created by compression rather than by passive dilatation. Tscherning published over 100 scientific articles, including a book titled Physiologic optics, which has been translated into English.
 * Associated eponym:
 * Tscherning's ellipse: Term used in corrective lens design used to identify the combination of lens curvature and lens power which have minimal aberration.