Johannes von Kries
(1853–1928) was a German physiological psychologist
who formulated the modern "duplicity" or "duplexity" theory of vision mediated by rod cells
at low light levels and three types of cone cells
at higher light levels.
When von Kries was at Freiburg, he was called to succeed Professor Du Bois Raymond as chair of physiology at the University of Berlin, but he declined.
Von Kries has been called Helmholtz's "greatest German disciple".