The Nineteenth Century , No 182, Apr 1892 [‎80r] (164/244)

1892
COLOUR-BLINDNESS
649
of colour-vision by assuming that our colour-sensations depend on the
stimulation of tihree separate sets of fibres or sensitive layers in the
retina. The isolated stimulation of a single retinal layer would be
associated with, a colour-sensation of red, green, and purple respec
tively, and these three colours would therefore constitute the
fundamental colours. All other colours would be derivatives of these,
and their production would be explained by a simultaneous, but
varying, amount of stimulation of either two or three of the hypo
thetical retinal layers. Blue would, for instance, according to this
theory, result from a simultaneous stimulation of the purple and
green layers.
The alternative theory, which is that of Hering, assumes that
the colour-sense proper can be best accounted for by assuming the
existence in the retina of two sensitive layers, each of which shall be
subject to modification in two opposed directions by a corresponding
pair of contrast colours. On this system, therefore, we obtain four
fundamental colours—green, red, blue, and yellow 1 —the first two and
the last two of these being associated together in pairs.
The phenomena which lead to the assumption of this association
of the fundamental colours in pairs are, primarily, the facts connected
with colour-after-images, and further the fact that the colours
which constitute a pair of contrast colours mutually neutralise each
other and combine to form an element of colourless light. e find
as a consequence that no colour in Nature can be seen as both
red and green at the same time, while, on the other hand, a green may
be a blue-green or a yellow-green, and a red either a yellow-red or
a blue-red. The same principle holds, of course, with respect to the
other pair of contrast colours, the laundress, for instance, availing
herself of it freely to make her clothes white by adding blue to neu
tralise the yellow they contain.
Neither of these two theories of colour-vision has as yet received
the general acceptance of physiologists. In point of fact, the language
of either theory accommodates itself readily enough to the expression
of the more obvious phenomena associated with colour-vision, and
with regard to these we could hardly say that the one theory supplies
us with a simpler explanation than the other does. ^ hen, howevei,
we pass on from the region of the well-established phenomena to
those of a more subtle observation, we find each theory being put
forward in its turn as the only one in which adequate provision is
made for the incorporation of the more recondite phenomena of
colour-sense which had not yet been elicited at the time when the
theory was originally brought forward.
The facts which would turn the scale in these cases in favour of the
1 Herring's hypothesis assumes the existence of two other contrast colours, black
and white, in addition to the four here enumerated. For our present purposes we
may, however, leave this last pair of contrast colours entirely out of consideration.