Biology Reference
In-Depth Information
This hypothesis may be said to supply a profound new insight
and deserves to be ranked as
the fourth major paradigm shift
within vision research
. Thus, based on the epoch-making contribu-
tions of Boll and Kühne, it soon became generally accepted that the
phototransduction, in both rods and cones, was photochemical in
nature.
3.5 Boll: discovery of rhodopsin as a visual
photopigment
As is sometimes the case, a great, new scientific discovery may be
promoted by an unexpected observation brought about by accident.
Franz Boll's marvellous discovery is an example. Thus, in Berlin in
June 1876, he was to demonstrate the previously known red colour
of the retina for the great Herman von Helmholtz among others. For
this, he used frogs. He killed them by cutting off their heads. He then
prepared the eyeball, bisected it with a pair of scissors, and used a pair
of tweezers to pull out the retina. However, in spite of this laborious
undertaking, he was not able to demonstrate the red colour of the
retina until more than half a dozen animals had been killed. This
unexpected result must have been of great concern.
Yet, five months later, in November 1876, the reason for the
failure became apparent. He had discovered that the red colour of the
retina in living animals was bleached in daylight and reappeared in
darkness - before his demonstration in June 1876, the frogs had been
kept in medium daylight (see Boll,
1877
).
Furthermore, he discovered that the bleaching and regenera-
tion of the red photopigment occurred in all animals with a retina
containing rods; in cold-blooded and warm-blooded, vertebrates and
invertebrates. Moreover, by using microscopy he found that the red
colour was situated in the outer segment of the rods.
After these epoch-making discoveries, he could conclude that
the red photopigment of the retina (later termed
visual purple
or
rhodopsin
) had to play an important role in visual processing. In his
own words,
