Hermann L. F. Helmholtz (1821-1894)
German physiologist, physicist, and psychologist. He studied medicine and served as a surgeon in the Army for a while, and later taught physiology and physics in Bonn and Berlin. He was interested in many subjects, ranging in the three fields already mentioned.
In 1847, he published the famous paper "On the conservation of force" which was to be regarded as one of the first fomulations of the law of conservation of energy. After this, he made physiological and psychological research on perceptions, visual and auditory, as well as epistemological research on geometry, and research on physics.
In 1921, Paul Hertz (physicist) and Moritz Schlick (philosopher of science) edited a selection of Helmholtz's papers, with their commentaries, as a centenary volume: Schriften zur Erkenntnistheorie. This volume had an impact on logical empiricists such as Reichenbach and Schlick himself, and Reichenbach in particular regarded Hermholtz as one of the major contributors to the modern philosophy of space and time, against such views as Kant's doctrine of the synthetic a priori. In addition to pointing out the importance of the notion of "congruence" (in terms of rigid rods) for physical geometry, Helmholtz also utilized his good knowledge of physiology and psychology for discussing the question of "visualization" of geometries, including Non-Euclidean geometries.