Herman Von Helmholtz, German physicist, c 1860.
© Science Museum / Science & Society Picture Library
Engraving by C H Jeens after a photograph. Helmholtz (1821-1894) studied a very broad range of subjects, including physics, physiology, and mathematics. He made discoveries in electromagnetic radiation, the mechanism of hearing and the speed of nerve impulses, as well as in optics for which he is best known. He published 'On the Conservation of Force' aged 26. His interest in physiological optics and colour vision led to his invention of the ophthalmoscope. This was used for studying the human retina, and paved the way for great advances in the study of eye diseases. His pupil Hertz discovered radio waves in 1888.