The Nineteenth Century , No 182, Apr 1892 [‎83v] (171/244)

THE NINETEENTH CEN April
The second objection which has been taken to Hering's theory
deals more particularly with the phenomena of colour-blindness. "W e
shall see that it is based upon a very careful study of these pheno
mena which has elicited that there are two varieties of the common
colour-blindness with which we have been occupying ourselves. It
is now asserted that in one of these varieties of colour-blindness the
defect in vision consists in an inability to see reds, while m the other
variety there is a similar incapacity to distinguish green colours.
These forms of colour-blindness have, therefore, been designated re
spectively as ' green-blindness' and 'red-blindness.' We can appreciate
at once how these designations have in course of the controversy
proved a tower of strength to the defenders of the Young-Helmholtz
hypothesis.
Here now, at any rate, there seemed to the onlookers to be some
flaw in the Hering theory of colour-blindness, for that theory asserts
the ordinary colour-blind to be all equaUy green- and red-blind, and
to belong essentially to a single category. To Professor Hering it
seemed, however, to be plain that, if there were differences which
allowed of the colour-blind being sorted into two classes, those dif
ferences must, in all probability, depend upon some differences in the
susceptibility to yellow and blue light on the part of those whom
his theory pronounced to be both red- and green-blind, further^
he argued that these differences, if they actually occurred, "would
probably not be limited to the colour-blind, but might be expected
to occur also in the normal-sighted.
In order to put this view to the test. Professor Hering undertook
a series of observations upon three normal-sighted persons—namely,
upon himself, and upon his two assistants. Dr. Biedermann and Dr.
Stilling. These experiments were designed to elicit whether any
constant differences could be detected in the colour judgments of the
three normal-sighted persons who were the subject of experiment.
The question proposed for judgment was the determination of the
point at which a red, which was graduated off on the one side into a
blue-red and on the other into a yellow-red, could be regarded as at
the neutral point at which it did not incline either to the one or other
of these colours. When the matter was put to the experimental test
in this manner, constant differences were actually discovered to obtain
between the judgments of the three individual observers. The one
observer, Dr. (now Prof.) Biedermann, in all cases still continued to
see a yellowish tinge when the red proposed for judgment had already,
in the judgment of the other two observers, long ceased to contain
any trace of yellow. Similarly, when it was a question of transition
from a blue-red to a pure red, the blue faded out from the red first
to Dr. Biedermann, next to Prof. Hering, and last of all to Dr. Stilling.
In fact. Dr. Biedermann had regularly begun to see a yellow shade in
the red before it had well ceased to have a blue shade for Dr. Stilling.