Biology Reference
In-Depth Information
his arguments in any detail, however, probably because he found the
explanations based on his model to be straightforward.
3.10 König: rhodopsin is the mediator
of night vision - a conclusive proof
Parinaud's investigation of nyctalopia strongly indicated that
rhodopsin was the photopigment responsible for night vision in
humans. Even more compelling evidence in support of this sugges-
tion was provided by König (
1894
). Using a spectrophotometer, he
measured the absorption coefficient of the rhodopsin of the human
eye over the spectrum from 640 nm to 420 nm and then compared the
spectral absorption obtained with the spectral sensitivity measured
psychophysically under scotopic conditions, in trichromats, dichro-
mats and rod monochromats. The values of the two measurements
corresponded excellently between 640 nm and 500 nm. Between
500 nm and 420 nm the spectral sensitivity was somewhat less than
that calculated from the spectral absorption curve. This discrep-
ancy could, however, be reasonably accounted for by the absorption
of the macular and the lens pigments of the eye.
3.11 The duplicity theory of König
Having provided conclusive evidence that rhodopsin mediated night
vision, König (
1894
) proceeded to formulate his special version of the
duplicity theory by seven simple theses as stated below:
1.
There is no visual purple in the central fovea or in any cone receptor.
(The same conclusion had previously been reached by Kühne,
1877b
,
1879
.)
2.
The colourless sensation obtained extrafoveally at the absolute
threshold for all wavelengths (except for red-related wavelengths) is
due to the decomposition of a small amount of rhodopsin.
3.
Stronger decomposition of rhodopsin produces visual yellow, which is
responsible for the primary blue colour sensation.
4.
The unknown visual substances responsible for the 'red' and 'green'
primaries are (like visual yellow) less sensitive to light than rhodopsin.
