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
K. 363, 364.] 
II. Theories of Vision 
low, power of responding to long waves of light. But a normal eye never 
responds poorly to the long waves in the way the protanopic eye does. Persons 
with trichromatic vision who have trouble in connection with the long waves 
are protanomalous. Similarly, there is a kind of trichromatic vision which 
differs from normal vision in another way, but only in a limited sense, and 
which is not characterized by particular sensitivity to long waves, or by low 
sensitivity to the short. This difference however, is associated with deteriora¬ 
tion in colour vision and leads by continuous transition to the deuteranopic 
There are therefore not two kinds of normal colour vision between which 
there is the same kind of difference as between the two kinds of dichromats. 
But there is only one normal colour vision which is capable of two modifica¬ 
tions. One of these is characterized by “poor use of long waves/7 to borrow 
this expression once; whereas this is not the case with the other. But both 
involve derangements of colour vision and may lead to dichromatic vision. 
At present, therefore, there can be no doubt as to the failure of the 
four-colour theory at this point, and that, in particular, the explanation 
which Hering tried to give for the two kinds of colour blindness is not 
compatible with the facts. Of course, this does not imply that the 
assumptions of the theory, especially the original fundamental proposi¬ 
tions of the theory, may not be right in a sense. But it is certain that 
the anomalies of colour vision point unavoidably to some set of 
relations that are not considered in the theory, and that thereby 
necessitate a limitation or extension of it. 
The theory of opponent colours has been just as unsuccessful in 
explaining total colour blindness. Originally, Hering believed that 
in these cases the organ of vision was without both coloured visual 
substances, and was therefore limited to the black-white substance 
only. Thus he believed that by ascertaining the stimulus values for 
such an eye the white valences would be determined; and that a con¬ 
firmation of this assumption can be found in the fact that the normal 
organ of vision in the state of dark adaptation in dim illumination 
(below the colour threshold) has very nearly just this same kind of 
vision. Apart from the fact that numerous peculiarities of the totally 
colour-blind eye would be completely incomprehensible from this 
point of view, this conception is shattered by the fact that these 
white valences may be totally different for two kinds of light which 
look alike to dichromats under the conditions of daylight vision; and 
also by the fact that, although in strong light the extreme periphery 
of the light-adapted retina has no colour vision anywhere, it can dis¬ 
cern differences of luminosity; and chiefly by the whole series of 
facts which force us to regard twilight vision as a function of some 
special constituent of the organ of vision.1 
1 The assumption that the effects on the black-white visual substance corresponds 
to the twilight values under all circumstances was found to conflict in some ways also with 
the luminosity distribution in the coloured spectrum. Hering tried to get around it in his


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