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. 376, 377.] 
II. Theories of Vision 
As to the main result of the more recent investigations, the writer 
would not venture to state that they are in harmony with any definite 
theory of the organ of vision or tend to give it support. The chief 
outcome has been rather to establish the fact that the different 
mechanisms of vision, which are different also in their efficiency, are 
distinguished from each other in very characteristic fashion by the way 
in which they react to different kinds of light, that is, by the peculiarity 
of the lights that appear to be the same. The thing to do, therefore, 
is to find out what lights look alike to each mechanism, because, as far 
as we know at present, this is the only perfectly unobjectionable 
method of obtaining information in comparatively simple fashion as 
to the efficiency of an eye in recognizing and discriminating colours. 
The reason why Königes researches and the vast amount of work 
that has been carried on in methodical connection with it have had 
such valuable results is just because, without any theoretical pre¬ 
sumption whatever, a certain group of facts was investigated that 
could be observed directly. And in this way results were obtained 
which may be regarded, also without auy theoretical bias, as general 
facts as to the mode of behaviour of the different mechanisms of vision, 
as found by direct observation. It is worth emphasizing this chiefly 
on account of the great practical value of these tests of colour vision 
for railway employees, etc. Just such conditions constitute the basis 
of methods of research, as has been stated before. But these facts 
must be further emphasized because, although it was formerly sup¬ 
posed that colour matches were particularly adapted for determining 
the character of vision, for a long time the method was viewed with 
some doubt, the tendency being to relegate it to a position of subordi¬ 
nate importance. As a matter of fact, Hering’s theory of the relative 
blueness or yellowness of vision as a peculiarity that may belong to 
the normal eye and the “red-green-blind” eye entirely in the same way, 
necessarily led to the conception, which is certainly very prevalent, 
that we are concerned here wdth peculiarities of comparatively sec¬ 
ondary importance, that are of no great scientific interest and certainly 
without any practical value.1 On the contrary, we must emphasize 
that this whole theory has proved to be actually incorrect. The 
differences in the normal eye are not of a similar kind to those between 
1 Hering’s vacillating attitude is responsible for this opinion in great measure. In 
his work on the subject published in 1885 he left it undecided whether “the division of the 
red-green-blind into red-blind and green-blind, which is considered more accurate by 
many persons, can be said to be correct, inasmuch as the higher degrees of blueness or 
yellowness of vision are more frequent than the medium degrees.” Thus he did not think 
it worth the trouble to come to a definite conclusion on this question. Subsequently, 
so far as the writer is aware, neither Hering himself nor any of his pupils made any ob¬ 
servations of this kind on dichromatic eyes or eyes with poor colour vision.


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