The Nineteenth Century , No 182, Apr 1892 [‎85r] (174/244)

1892
COLOUR-BLINDNESS
659
own methods, and a mere glance at the signal lamps of almost any of
our railways, or at the side lights of our ships, will show that the dis
crimination between the red and the green which are employed
would be within the competence of anyone who could see a difference
between blue and yellow. As a matter of fact, we find the red glass
which is brought into requisition has a very appreciable tinge of
yellow, and the green has almost as pronounced a shade of blue.
These differences, which put the discrimination of the lights within
the competence of the colour-blind, are, however, probably due rather
to an accident of convenience of manufacture than to any scientific
intention, for it will be noticed that the coloured glass employed in
omnibuses and cab lamps generally conforms to the same type.
When we have once realised how the discrimination between the
red and the green of the signal lights .comes to be under present con
ditions within the competence (though of course often only very
iiarrowly within the competence) of the colour-blind, we see that it
would be possible to put a stop to many of the accidents which still
creep in under our present system of examinations, by taking the
precaution of bringing the signal colours in all cases comfortably
within the competence of the colour-blind, by providing that a dis
tinct orange should replace the red which is now in use, and that a
more pronounced shade of blue should be introduced into the green
of the signals. In that case, we might perhaps be able to dispense
with our colour-vision examinations.
We must remember that we can never be perfectly safe against
accidents which are due to colour-blindness by any system of admis
sion examinations, and this for two reasons. First, because colour
blindness is not necessarily congenital, but may originate, as we have
seen, at any time. Secondly, because it is at all times difficult in
testing for colour-blindness to be quite certain that the colour-blind
person will not evade detection by appreciating some accidental
difference of illumination, or of blue and yellow colouring which we
may have failed to notice in the colour-vision tests we are proposing
to him. We have seen that the ' practical tests' open the door wide to
such a contingency. If we, then, upon the basis of a series of successful
discriminations, pronounce an examinee to be free from defects of
colour-sense, we admit him to employment where, in the existing
condition of things, the differences between the signal lights are to
him of so subtle a character that some fine day, when perhaps the
conditions of light are unfavourable to him, he may fail to make the
discrimination and a catastrophe ensues. There are two ways in
which this condition of things could be remedied. The first would
be by the installation of yellow and blue as signal colours in the
place of red and green. This substitution would, from the point
of view of the theory of colour-vision, evidently be the ideally perfect
one, since we should then be employing for signal purposes colours