Bauhaus-Universität Weimar

Titel:
Helmholtz's treatise on physiological optics. Volume 2. Edited by James P. C. Southall. Translated from the 3rd German edition
Person:
Helmholtz, Hermann von
PURL:
https://digitalesammlungen.uni-weimar.de/viewer/image/lit39650/277/
225.] 
§24. Contrast 
265 
colour alongside another colour resembles more or less the complemen¬ 
tary colour of the latter. The opposition thus manifested is implied 
in the term contrast. Chevreul draws a distinction between simul¬ 
taneous contrast, as applied to the phenomena belonging here, and 
successive contrast, where two colours appear in succession upon the 
same retinal area. 
However, cases also occur in which the colour of a part of the visual 
field is so altered by being adjacent to another colour that it becomes 
similar to the latter itself, and not to its complementary colour; and 
in these instances the term “contrast” might not seem to apply so 
directly, although, perhaps, as a matter of fact the alteration of one 
colour here is by a contrast with the complementary to another colour. 
So as to include these cases also, Brücke calls the colour that is evoked 
by the action of one existing adjacent to it in the visual field, the 
induced colour; and the one that is responsible for the appearance of 
the other, the inducing colour. And so when the field, whose colour is 
altered, is itself coloured, we shall speak of this colour as the reacting 
colour, as formerly. The alteration of the reacting colour by the 
induced one leads to what may be called the resulting colour. In 
general, therefore, the idea of contrast is not directly appropriate 
except in the ordinary cases where the induced and inducing colours 
are complementary. But there are instances where the induced colour 
is identical with the inducing one. 
The phenomena of successive contrast, which will be considered first, 
are easily comprehended from what has been stated in the previous 
chapter. After looking at a field of colour A and medium brightness, 
suppose the eye turns to look at another field of colour B. Then as a 
rule, the residual stimulation of the impression A will not be strong 
enough for a positive after-image to be projected on a second field of 
medium brightness; and so there will be a negative after-image of A 
upon the field B. Thus those parts of the colour B that are like A will 
be diluted. If B is of the same hue as A, it becomes whiter by contrast; 
if it is complementary, it becomes more saturated. If it lies on one side 
or the other of the colour circle between A and its complementary 
colour, it changes into an adjacent hue farther from A and nearer the 
complementary colour. Incidentally, the brighter A was, the darker B 
looks. Accordingly, this would be the general law of successive con¬ 
trast, on the supposition that the luminosities of the two fields were 
such that only negative after-images could occur. 
Even in comparing coloured areas with each other that he side by 
side in the visual field, successive contrast, that is, contrast caused by 
after-images, is a very important factor, as any one can easily verify.
        

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