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representing the three different primaries of the Young-Helmholtz
colour theory - the cones appeared essentially similar. Hence, he had
to conclude that these retinas contained only one type of cone. Yet, he
could point to another characteristic of the cones that might account
for the triplicity of colour vision: each cone had several outgoing fibres
connected to the inner segment, while the rod receptors had only one
fibre connected. Reasoning from the plausible assumption that each
cone needed several fibres in order to handle both achromatic and
chromatic information processing, while a rod receptor, mediating a
simple achromatic sensation, needed only one fibre, he came to the
conclusion that the different fibres connected to each cone receptor
were capable of reacting in a qualitatively different manner when
excited by light of different colours (Schultze,
1866
, p. 258). Thus,
Schultze, in effect, suggested that the different fibres connected to
the inner segment of each cone in mammalian and fish retinas consti-
tuted the basis for colour vision, serving independent, qualitatively
different nerve processes.
In the retinas of diurnal birds, on the other hand, he
did
ind
evidence for three different kinds of cone receptor. Thus, he found
cones with red, yellow and colourless droplets (globules) situated at
the junction of the inner and outer segments, so that light had to
penetrate them before entering the outer segments. The droplets,
therefore, appeared to function as colour filters, and Schultze came to
the conclusion that these different cone receptor types represented,
respectively, the red, green and violet receptor mechanisms of the
Young-Helmholtz colour theory. As an alternative, he suggested that
cones with colourless droplets might function in a similar way to
the cones of humans, being capable of generating the whole colour
gamut.
The interesting finding that the outgoing fibre of the cones
with coloured droplets was relatively thin and slender, like that of
the rods, was interpreted as consistent with his prediction that each
cone needed only one fibre when the retina contained several types
of cone receptor.
