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and that some of the properties of the primary 'blue' mechanism had
features in common with the ordinary dark-adapting rods. Thus,
for example, he noted that while deuteranopia and protanopia were
inherited as sex-linked allelomorphs, tritanopia (blue-blindness) was
inherited quite separately as an autosomal recessive condition. Thus,
the genes for the development of the structures responsible for the
'blue' mechanism, like those of the rod mechanism, were carried on
a different chromosome from the genes for the 'red-green' complex.
He also noted that the Weber fractions (that is, the ratio of a just
detectable increase in intensity to the intensity of the stimulus)
measured for the 'blue' and the rod mechanisms were strikingly
similar and so were their receptive field sizes.
More substantial evidence was found in the results of Trezona
(1960) that a positive, blue afterimage observed in darkness after a
strong 'white' bleach may be readily obtained just outside the fovea
where rods are activated, but not in the rod-free central fovea (it should
be noted that this positive, blue afterimage appears akin to the so-called
positive, complementary Purkinje afterimage, discussed by von Kries,
1929
). Willmer (
1961
) also pointed to the fact that blue, violet and
purple colours in many instances were associated with twilight vision.
Yet, Willmer's most convincing evidence appears to be some
remarkable results he obtained in an experiment on simultaneous
contrast, where he found that an inducing field of long wavelength,
assumed to activate mostly cones, could generate a blue colour in a
test field assumed to activate only rods. The same blue colour was
observed independently of both the test wavelength (varied between
400 and 580 nm) and the inducing wavelength (varied between 580
and 700 nm) (see Willmer,
1950
).
9.1.5 Evidence opposing rods as 'blue' primaries
Finally, in order to complete his argument, Willmer (
1961
) scrutinized
objecctions that could be raised against his basic assumption that
rods may represent the primary 'blue' mechanism under photopic
conditions:
