Allvar Gullstrand and the slit lamp 1911.

The Swedish ophthalmologist and self-taught mathematician Allvar Gullstrand (1862-1930) invented the slit lamp to illuminate the anterior of the eye. With its rectangular beam of very bright light, he studied the structure of the cornea and the function of the lens. His dioptric investigations showed that, as well as the extracapsular mechanism described by Helmholtz, changes in the substance of the lens, that he termed intracapsular, also contribute to accommodation. However, his invention has been appropriated by clinical ophthalmologists and is now routinely used in examination of the eye.