Adolf Eugen Fick (1829-1901) was a German physician and physiologist. He began his work in the formal study of mathematics and physics and then realized an aptitude for medicine. In 1855 he introduced Fick's law of diffusion, which governs the diffusion of a gas across a fluid membrane. In 1870 he was the first to measure cardiac output, called the Fick principle. Fick invented the tonometer. Tonometry is the procedure eye care professionals perform to determine the intraocular pressure (IOP), the fluid pressure inside the eye. His nephew was German ophthalmologist Adolf Gaston Eugen Fick. He lived to be 72 and died of natural causes.