Helmholtz’s Phakoscope

Between 1840 and 1870, the study of vision underwent a profound transformation, achieving a level of methodological and theoretical integration previously unattained. At the forefront of this turning point was the polymath Hermann von Helmholtz. This paper examines Helmholtz’s role in the emergence of the new field of physiological optics, highlighting his laboratory experiments that integrated classical optics, physiology, and experimental psychology. As a case study, it illustrates an instrument of his invention, the phakoscope—a device designed to demonstrate the mechanism of accommodation of the crystalline lens—which became widely used in nineteenth-century physics, physiology, and experimental psychology courses. Following an introduction of the instrument and its educational applications, the study unveils crucial insights into the operational method and limitations of the original configuration through a reenactment of Helmholtz’s observations using one of the last surviving original devices, preserved at the Deutsches Museum, Munich.