Bauhaus-Universität Weimar

Helmholtz's treatise on physiological optics. Volume 2. Edited by James P. C. Southall. Translated from the 3rd German edition
Helmholtz, Hermann von
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.)


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