Biology Reference
In-Depth Information
Apparently, these facts were derived mainly from his own
painstaking histological studies and Aubert's (
1865
) demonstration that
saturation of colours deteriorated during long-term dark adaptation.
Conclusive evidence that only rod receptors function in night vision
was later provided by König (
1894
), who demonstrated that the spectral
absorption of the photopigment of rods (rhodopsin) closely coincided
with the spectral sensitivity of the human eye in night vision.
Perhaps the best illustration of how an observation may
trigger a paradigm shift, though, was the unexpected discovery by
Boll (1877) that the photopigment rhodopsin in the outer segment
of the rods bleached in light and regenerated in darkness. Boll
soon understood that rhodopsin had to play an important role
in visual processing and therefore started an investigation of its
photochemical reactions to reveal its secrets. This research was
quickly followed up by Kühne who, during an intense research
period (1877-1882), arrived at his general photochemical theory
of vision, where phototransduction both in rods and cones was
explained in photochemical terms. Accordingly, he postulated the
existence of photopigments, not yet measurable, in the cones. In
fact, he suggested that even rods in the light-adapted state, where
rhodopsin was substantially bleached, contained undiscovered,
visually active photopigments.
The trichromatic colour theory was also instigated by an
observational fact. Thus, the trichromatic colour theories of
Lomonosow, Palmer and Young were all based on the old and
well-known fact that three different pigments mixed in various
proportions were sufficient to produce all object colours. This
fact, however, did not bear much weight, since, as Newton
(
1730
) and later Helmholtz (
1852
) had made clear, the colour
produced by pigment mixing depended on a subtractive mixture
procedure making it impossible to control the kind of rays that
struck the retina. Also, Helmholtz (
1852
) found that adding
three
spectral
lights could not produce all spectral colours, and
