Biology Reference
In-Depth Information
Since von Kries was recognized as the leading authority on the
duplicity theory, and the 1929 paper gives his final and most thorough
defence of the theory, we will present this defence in some detail.
The main evidence supporting the duplicity theory was discussed
under three major headings:
1.
Lights that match in day vision may differ in twilight vision: the Purkinje
phenomenon.
Here, he presented experimental results which showed that a colour
match between two light patches, obtained extrafoveally in a light-
adapted state, may break down when the intensity of the two fields is
lowered equally and the eye becomes dark adapted. Indeed, he showed
that a colour match obtained in a light-adapted state at mesopic
luminance levels may break down by dark adaptation without any
change in light intensity.
As noted above, both of these observations were opposed to
Newton's additivity law of colour mixture. With a suggestion similar
to that of König (
1894
), von Kries explained the apparent additivity
failure by the assumption that there were two different modes of
vision, a chromatic day vision and an achromatic twilight vision,
and that, as the mode of vision gradually changed from day vision to
twilight vision, the colour field with the strongest twilight activation
would gradually become relatively more desaturated and bright.
Obviously, this explanation implies that the two modes of
vision may function simultaneously in an overlapping intensity
range (the so-called mesopic intensity range), and that an increase of
rod activity in a test field may reduce the saturation and increase the
brightness of the test colour - all in agreement with Schultze (
1866
),
Parinaud (
1884b
,
1885
) and König (
1894
).
Strong support of the duplicity theory was also found in the
observation that the breakdown of colour matches could be obtained
in all parts of the extrafoveal retina, from the parafovea to the
extreme periphery, but not in the central fovea where only cones
were operating, and that only achromatic colours could be seen in
twilight vision.
