The Nineteenth Century , No 182, Apr 1892 [‎82r] (168/244)

1892
COLOUR-BLINDNESS
653
l)roug]it out by the fact that, though a not inappreciable fraction of
mankind have been colour-blind, our first description of the condition
dates only from the last century, when John Dalton discovered, and
then only after years of self-observation, that red and green were names
for sensations which had no existence for him. It is, on the other hand,
true that a colour-blind person is sometimes in ordinary life at a dis
advantage as compared with a person with normal colour-sense. He
is at a somewhat serious disadvantage, for instance, when he is set
down to pick strawberries in a garden, for the ripe bluish-red straw
berries play a kind of game of hide and seek with him among the
bluish-green leaves which, as we have seen, must be of indistinguish-
ably the same colour to him. In the ordinary method of testing for
colour-blindness by means of Holmgren's wools^the oculist avails him
self of colour-mixtures which are quite comparable to these in order
to place the colour-blind at a disadvantage.
Having thus obtained, it is hoped, the beginnings of a true under
standing of colour-blindness, we have to continue our inquiry by
endeavouring to arrive at some kind of a notion of what the colour-
blind really see where we see ordinary green and red. In order to do
this we shall have to begin by a preliminary analysis of our every-day
colour-sensations. In point of fact, we find when we attend to the
matter that our familiar sensation of, for instance, red and green can be
resolved into three component sensations, which become agglomerated
together in our minds into a single sensation simply in consequence
of the fact of their being simultaneously received on the retina.
We have, first, a sensation of white or colourless light, which, as it
were, dilutes the green or red, and converts it into what is technically
known as ' unsaturated colour.' We have further in every ordinary
green and red (for we are not dealing with theoretically pure colours)
an element of either blue or yellow. And, lastly, we have the residual
and essential element of the sensation, which in strictness is the only
element denoted by the term ' red' or ' green.'
We have already dealt with the first two elements in the complex
sensation in their relation to colour-blindness, and we have seen that
the colour-blind are entirely competent with regard to them. Our
present task is, therefore, limited to endeavouring to elicit what the
single undifferentiated sensation is which corresponds in the colour
blind to the dual red and green sensations of the normal-sighted. In
other words, we have to seek to arrive at the connotation which the
colour-blind attach indiscriminately to the terms green and red.
This is evidently an almost impossible task, but we may perhaps
note a few facts which will enable us to arrive at some notions on this
very difficult subject.
To begin with, it has been ascertained that a colour-blind person
is not blind to a red element intermixed with another colour, in at all
the same way as if he were really p a blind person. Neither,
V ol. XXXI —No. 182 Y Y