Project GutenbergGerman physicist and physician Hermann von Helmholtz was born #OTD in 1821.
His 1847 work, Über die Erhaltung der Kraft, articulated the idea that energy cannot be created or destroyed, only transformed from one form to another. In physiology, he invented the ophthalmoscope in 1851. His book On the Sensations of Tone as a Physiological Basis for the Theory of Music (1863) explored the physical basis of musical harmony and the physiology of hearing.
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