434
The Sensations of Vision
[360, 361. K.
The somewhat more complicated conditions of anomalous trichromatic
vision can be made to fit in it in an intelligible manner, although not
quite so simply. And it affords us at least the simplest way of ex¬
pressing the observed facts in a comprehensive system. If, on the
other hand, it has to be stated that it certainly is not a definitive
theory of the whole of vision, as Helmholtz supposed, there can hardly
be any doubt as to the correctness of its fundamental conceptions. A
rather more general discussion of theoretical questions will be deferred
until other theories and other groups of facts have been described.
2. Other Theories of the Sensations of Light and Colour
While Helmholtz deemed it illegitimate or at least untrustworthy
to draw conclusions as to physiological processes from the direct
psychological character of the sensations, most subsequent theories
of vision use this very group of facts as their starting point. The
results of this mode of treatment are in harmony as to their main
features at least, and so all these theories have a common stamp im¬
pressed on them. In the first place, they assign a very special im¬
portance to the series of colourless sensations ranging in all degrees from
black to white.1 A similar importance is also assigned to certain colours,
which are accordingly referred to as pure, simple or primary colours,
namely, red, green, yellow and blue.2 Another thing to be mentioned
about it is that in sensation each quality of one pair seems capable of
being combined with both qualities of the other pair (that is, red with
blue as well as with yellow and yellow with red as well as with green) ;
but the two determinations of each pair are mutually destructive,
that is, red and green cannot be combined, nor yellow and blue.3 This
conception is a very old one fundamentally. Of late years it has been
developed and advocated chiefly by Aubert, and may be called the
four-colour theory.
Unfortunately, this theory has been very inadequately tested
as to the very points that are capable of being easily investigated.
Obviously, it would be important to find out what objective lights
can, under proper conditions, give the impression of “pure” colours,
and whether in this respect various observers would agree or disagree.
Investigations of this kind are very scarce; and among the advocates
1 ^Sometimes called “toneless” or “untoned” or “grey” sensations as distinguished
from the “toned” or coloured sensations, namely, red, yellow, green and blue; as mentioned
in the text. (J. P. C. S.)
2 TfThe so-called “psychological primaries.” This is another instance of the confusion
of colour terminology in physics and psychology. (J.P.C.S.)
3 ^These latter combinations are what Mrs. L add-Franklin terms colour-fusions or
colour-extinctionsy to which attention was called elsewhere. (J. P. C. S.)