Biology Reference
In-Depth Information
view that visual sensations depended on photochemical processes,
i.e. the initial reaction in the retina is the absorption of light by a
photochemical see-substance. However, the speculation of Boll
(
1878
) that the photochemical reaction of rhodopsin might constitute
the basis for colour vision was rejected, since he found no rhodopsin
in the human central fovea, where colour vision is best developed
(Kühne,
1877a
, p. 32,
1877b
, p.107). In fact, he found no rhodopsin in
cones, despite extensive investigations in different species including
snakes, in which the retina contained only the cone receptor type
(Kühne,
1877b
,
1879
).
Consequently, Kühne drew the conclusion that there had to be
several photochemical substances in the retina, not just rhodopsin.
Indeed, he suggested that the rod receptors, like the cones, contained
colourless photochemical substances that could promote vision under
conditions where the rhodopsin had been completely decomposed
(Kühne,
1879
, pp. 329-332). He found supporting evidence in that
animals (rabbits) apparently with a pure rod retina could see even
when rhodopsin had been bleached away.
Yet, in harmony with Schultze's duplicity theory, Kühne suggested
that rhodopsin in rods represented the photopigment of twilight vision,
only giving rise to achromatic sensations. Thus, he stated 'dass wir
mittelst des Purpurs und der Stäbchen (ohne die Zapfen) das Spectrum
nicht nur wahrnehmen, sondern auch in Grau schattirt, ähnlich wie
der Farbenblinde, auffassen würden' ('that not only would we see the
spectrum by means of the rhodopsin and the rods (without the cones)
but also see it in shades of grey like the colour-blind'), and further,
Unsere Erfahrungen über das Vorkommen und Verhalten des
Sehpurpurs sind so sehr in Uebereinstimmung mit M. Schultze's
Hypothese von der physiologisch-chromatischen Bedeutung der
Zapfen und Stäbchen, dass es nur des Hinweises darauf bedarf
um alle Hoffnungen herabzustimmen, die man etwa auf die
Optographie im Sinne specifischer Farbenwahrnehmung setzen
mochte (Kühne,
1877a
, p. 93).
