German physiologist, physicist, and inventor of the ophthalmoscope for examining the inside of the eye. He was the first to explain how the cochlea of the inner ear works, and the first to measure the speed of nerve impulses. In physics he formulated the law of conservation of energy, and worked in thermodynamics.

Helmholtz's scientific work in many fields was intended to prove that living things possess no innate vital force, and that their life processes are driven by the same forces and obey the same principles as nonliving systems. He arrived at the principle of conservation of energy 1847, observing that the energy of life processes is derived entirely from oxidation of food, and that animal heat and muscle action are generated by chemical changes in the muscles.

Helmholtz was born in Potsdam and studied at the Friedrich Wilhelm Institute in Berlin. He first became professor at Bonn 1855 and ended his career as director of the Physico-Technical Institute of Berlin from 1887. Helmholtz invented the ophthalmoscope, which is used to examine the retina, 1851, and the ophthalmometer, which measures the curvature of the eye, 1855. He also revived the three-colour theory of vision first proposed 1801 by English physicist Thomas Young, by showing that a single primary colour (red, green, or violet) must also affect retinal structures sensitive to the other primary colours. This explained the colour of afterimages and the effects of colour blindness.

In acoustics, Helmholtz produced a comprehensive explanation of how the upper partials in sounds combine to give them a particular tone or timbre, and how resonance may cause this to happen.