The emphasis of this chapter is my discussion of why and how the polymath Hermann von Helmholtz (1821–1894) at age 29 invented an optical instrument to visualize the retina in a patient’s eye. In modern usage this medical device is called an ophthalmoscope. He [Helmholtz] knew more physics than physicians did, and more physiology and medicine than physicists and mathematicians. Quote from Helmholtz in later life on the role of interdisciplinary education and training in support of his invention of the Augenspiegel. Helmholtz (1903). Vortrage und Reden von Hermann von Helmholtz, fifth edition. Braunschweig: Friedrich Vieweg & Sohn. There are two methods of ophthalmoscopy: the direct method of ophthalmoscopy with an upright image of the retina, and the indirect method of ophthalmoscopy with an inverted image. Helmholtz’s Augenspiegel uses the direct method of ophthalmoscopy. In 1852, Christian Georg Theodor Ruete, (1810–1867), an associate Professor of ophthalmology at the University of Göttingen, invented the indirect method of ophthalmoscopy; the indirect ophthalmoscope forms an inverted image of the retina.