Helmholtz, Hermann Ludwig Ferdinand von, a distinguished German physician and scientific writer, born in 1821. He was educated for the medical profession, and practised as a surgeon at Berlin and Potsdam. After having been an assistant in the Berlin Anatomical Museum, he held the chair of physiology at Konigsberg, Bonn, and Heidelberg successively, and in 1871 was appointed professor of physics at Berlin. His work in physics and in physiology is equally valuable. To the first belongs his essay on the Conservation of Energy (1847), also his analysis of the spectrum. In physiology he has devoted himself chiefly to physiological optics and the connection between sight and sound. He invented an instrument with which the retina of the living eye can be studied, and wrote Theory of Sound Sensations (1862), and Sensation of Tone as a Physiological Basis for the Theory of Music (trans. 1875). Besides this he made investigations on the subject of nerve disturbances, vibrations of air in pipes, and the relation between colour and sound. The English translation of his Popular Scientific Lectures (Vortrdge) has an introduction by Tyndall. He died in 1894.