The vision of Helmholtz.

Hermann Ludwig Ferdinand von Helmholtz (1821-1894) began investigating vision at a time when its study was undergoing a revolution. Laboratory experiments were augmenting the long history of naturalistic observations. Instruments of stimulus control enabled the manipulation of time and space in ways that had not been possible previously, and Helmholtz added to their tally. Vision was a central issue in his early years as an academic, and the bicentenary of his birth is here celebrated visually. Much of his research on vision was described in his Handbuch der physiologischen Optik, which was translated into English to mark the centenary of his birth. The history of his Handbuch is examined, together with illustrating highlights from it. Helmholtz's contributions to understanding the eye as an optical instrument, the sensations of vision, and perception were expressed in the three parts of the Handbuch, which became the three volumes of his Treatise on Physiological Optics.