The Nineteenth Century , No 182, Apr 1892 [‎82v] (169/244)

the nineteenth
April
on tlie other hand, does lie see an equivalent of white or colourless
light for what we see as coloured light. Rather he sees a something
which is not to us at present fully intelligible, and which indeed will
never be thoroughly understood by us unless it should turn out to be
possible to induce a similar condition of colour-blindness artificially
in ourselves. What the colour-blind person does see in these cases is
described byDalton—who, it will be remembered, was himself colour
blind—as a neutral tint, and that seems almost as far as we can go in
the matter of defining the colour.
We can, however, gather with a certain amount of probability
that this neutral tint, when it occurs either isolated or in combina
tion with yellow or blue, can be distinguished by the colour-blind
from either grey or an admixture of grey with other colours. This
is, however, only an inference from the fact that a majority at least
of the colour-blind seem, when they are dealing with greens or
reds, to be in some way apprised of the fact that they are dealing
with colours of which the further discrimination ought to be possible
to them. It is probably some such vague feeling of incompetence
in the matter of resolving them which seems to render particular
shades of colour positively disagreeable to certain colour-blind people.
It has, for instance, been noticed in connection with a certain
eminent colour-blind mathematician that he invariably proceeds, as
soon as he sits down to his table, to cover up and hide out of sight,
4 as disagreeable,' any books which may happen to be bound in a
particular, to the normal-sighted perfectly unobjectionable, shade of
green. We can perhaps succeed in placing ourselves to some extent
at his standpoint in this matter by fixing our eyes steadily upon
some bright or red or green object—let us say, upon a square of red
paper—and then awaiting the appearance of the after-image. We
must, however, for our present purposes beware of looking for the
after-image in the ordinary manner either by closing our eyes, or
by directing them away to vacancy, when we should of course see
the after-image of a green square, with which Pears's advertisements
must have thoroughly familiarised the readers of English magazines
of some few years back. We have instead to keep our eyes fixed
rigidly upon the red paper, and we shall soon find that one after
image will appear to us there. It is in this connection necessary for
us to realise that the after-image does not, as one might at first
suppose, spring suddenly into being in the instant in which the eyes
are removed from the paper. Eather, as Hering has shown, it begins
to come into existence as soon as our eyes are directed to the
object, and it then gradually works 4 its way up to our consciousness,
through a retina which is becoming more receptive to its influence.
Thus, in our present experiment, the after-image of green will, as it
4 This workirg its way throiigh of a sensory phenomenon to consciousness while