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/lit39650/298/
286 
The Sensations of Vision 
[242, 243. 
parent coloured veil, seems to be spread over the field. The immediate 
impression is not that this colouration is absent where there is white, 
that is, it is not just a mere substitution of the complementary colour of 
the ground in place of the white ; but the idea seems to be that two new 
colours are substituted in the place of the white, namely, the colour of 
the ground and the complementary colour. The connection is clearest 
in the arrangement shown in Fig. 54, where the observer looks through 
the green glass inclined at an angle of 45°. He decides that the black 
spot on the horizontal surface is pink-red, but he also decides that this 
spot, as well as the entire surface with its pink-red colour is seen 
through the green glass, and that the green colour given by the glass 
extends uninterruptedly over the entire lower surface, and even over 
the dark spot. Thus he believes that he sees two colours together at 
this place, that is, green, which he attributes to the glass plate, and 
pink-red, which he attributes to the paper behind it; and the two of 
them together do, in fact, give the true colour of this place, that is, 
white. As a matter of fact, an object which, seen through a green 
glass, sends white light to the eye, as this spot does, would have to be 
pink-red. But when a white object of exactly the same appearance is 
placed above the plate of glass, every reason for resolving the colour 
of the object into two disappears; it looks white to us. 
It is the same way when coloured surfaces are covered with translu¬ 
cent paper. If the ground is green, the paper itself seems to be greenish. 
Now if the substance of the paper extends without a perceptible break 
over grey underneath, the observer thinks he sees an object shining 
through the greenish paper; and an object of this kind must, on the 
other hand, be pink-red in order to give white light. But if the white 
place is outlined as an independent object, and there is lack of con¬ 
tinuity between it and the greenish part of the surface, it is regarded 
as being a white object lying on this surface. In §20 above, it was 
stated that this sort of separation of two colours that are present in 
the same part of the visual field is a matter of judgment. We were 
confronted with this condition there as something that was an ob¬ 
stacle to the free realization of the sensation of a compound colour. 
A separation of this sort is a very frequent occurrence whenever the 
two colours are unevenly distributed. These phenomena were noticed 
first by Volkmann,1 and he describes the effect by saying that we seem 
to see one colour through the other. In the author’s opinion the faculty 
of making such a separation depends on the following circumstance. 
Colours have their greatest significance for us in so far as they are 
properties of bodies and can be used as marks of identification of bodies. 
1 Müllers Archiv für Anat. und Physiol. 1838. S. 373.
        

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