Duplicity Theory of Vision: From Newton to the Present

11 Night vision may appear bluishThe origin of the idea that rods may give rise to chromatic sensationsmay be traced back to the hypothesis of Ebbinghaus (1893) that thephotopigment of rods (rhodopsin) was the yellow-blue see-substancepostulated by Hering (1878), and the suggestion of König (1894) thatthe rod receptors represented the primary ‘blue’ receptor systemof photopic vision. Substantial evidence in favour of the idea wasprovided by von Kries and Nagel (1896) who found that twilightvision contained a tint of blue.In order to identify more precisely this blue colour quality ofrods, von Kries (1896) attempted to find the spectral light that didnot change its hue when test intensity was reduced from photopic toscotopic levels, i.e. when the rod component increased. He used Nagel(a deutranope) as his subject and instructed him first to make a colourmatch at a photopic intensity level between a homogeneous spectraltest light from the short-wave region of the spectrum (and hence witha relatively high potential scotopic value) and a mixture of spectrumred (670 nm) and violet (435 nm) lights (and hence with a relativelylow potential scotopic value). Thereafter, the intensity of the testand comparison fields was reduced in the same proportion from thephotopic level until the homogeneous light strongly activated therod component. His results showed that the spectral light that didnot change in colour quality with intensity reduction was situatedbetween 480–485 nm, i.e. in the green-blue part of the spectrum.Dreher (1912) made a somewhat similar investigation usingtwo trichromats as subjects and confirmed the results of von Kries.G. E. Müller (1923), in his review on the colour quality of night vision,also concluded that, in addition to the dominant achromatic component,it entailed a tint of green-blue. He presumed that this green-blue colour