The Nineteenth Century , No 182, Apr 1892 [‎84v] (173/244)

658
THE NINETEENTH
April
would in such cases be discriminated against. In the condition of
deeply pigmented retina, blue-green would, therefore, be the most
difficult colour to recognise, because of the excessive elimination of
these rays during transmission, and in this manner we might, if it
were worth while, constitute a sub-class of the green-
blind. Similarly, we might constitute another sub-class of
red-blind by grouping together all the colour-blind persons whose
maculse transmitted more than an average of blue-green rays. We
must remember that the transmission of an excess of blue-green rays
would act as a set-off against the yellow-red rays, and would thus, by
neutralising their colour-properties, render these rays proportionately
difficult to appreciate.
With this we appear to have reached a stage in our study of
colour-blindness at which it would perhaps be justifiable to feel a
certain amount of confidence in the contrast-colour hypothesis theory
as, at any rate, a good working hypothesis. We may therefore,,
perhaps, now venture to approach the practical question of the pre
vention of accidents from colour-blindness, with some practical
proposals.
To begin with, we have to keep a firm hold of the fact that there
is no case of yellow-blue blindness on record. We have also to keep
before us the fact that the green-red colour-blind find their way
through life very comfortably with their single pair of colour-
perceptions, and with their power of distinguishing differences of
illumination, and that they exploit these to an extent which we
hardly realise in the discrimination of colours which the normal-
sighted discriminate more naturally by differences in the reds and
the greens. Now when a colour-blind person is to be tested in a
scientific manner to elicit the defect in his colour-sense, it is a matter
of duty to propose to him the solution of the only question which
his defect of colour-sense absolutely incapacitates him from solving.
We, therefore, as has been explained, set him down to discriminate
between red and greens from which we have designedly eliminated
all differences of illumination, and of blue or yellow colouration. He
naturally falls into the trap we have laid for him, and we proceed
then to reject him for railway- or sea-employment, without reference
to the fact that the problems there proposed to him for solution
may be quite within his competence.
This defect in our methods of examination has, it is true, been
to some extent remedied by the so-called ' practical tests'— i.e. by
proposing to a candidate for employment the solution of the actual
problems which he will be required to solve in the course of his work.
This method of testing cannot, and indeed does not, pretend to be
a scientific system of testing for colour-blindness ; for though we do
in point of fact ask the candidate to distinguish between red and
green, we allow him to undertake the discrimination entirely by his