Night vision may appear bluish - Duplicity Theory of Vision - page 110

was generated when light activated rhodopsin, but could not decide with
certainty whether the rhodopsin molecules generating the green-blue
colour were situated in the outer segment of rods or cones, although he
thought it most likely that the green-blue colour had a rod origin.
It should be noted, however, that there was no general agreement
as to the colour quality of night vision. Most subjects described night
vision as achromatic. Even von Kries ( 1896 , p. 87) found the word
'farblos' ('colourless') an appropriate description. Thus, in a footnote
he remarked,
Es wäre also vielleicht richtiger, nicht zu sagen, dass die Stäbchen
farblose Empfindungen, sondern dass sie einen nur einsinnig
veränderlichen Empfindungseffekt liefern. Da indessen der
Stäbcheneffekt sich schwerlich in erheblichem Masse von der
Farblosigkeit im gewöhnlichen Sinne unterscheidet, so schien
es mir besser, den obigen, seine Bedeutung jedenfalls sehr viel
anschaulicher kennzeichnenden Ausdruck beizubehalten.
(Thus it might be more correct not to say that the rods give rise
to colourless sensations, but that they give rise to one quality
only. Yet, since the rod effect does not differ much from what is
generally considered as achromatic sensation, it appears to me
better to retain the above expression, which, at any rate, gives a far
more elucidating characterization.)
The discrepancy between the reports of different subjects
with regard to the colour quality of night vision could most easily
be explained by individual differences. Yet, an alternative and more
fruitful hypothesis was offered by Nagel ( 1911 ). He assumed that the
colour quality of twilight vision, for each subject, may vary from
colourless to blue. Thus, he wrote, 'Incidentally, it is quite conceiv-
able that the light sensations that occur under the condition of pure
twilight vision have a certain range of fluctuation as to their quality,
varying from absolute colourlessness to a cyan-blue of no little satura-
tion' (Nagel, 1911 , p. 350). He assumed that this change in colour
quality was due to some unknown adaptation mechanism.