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/lit39650/279/
226, 227.J 
§24. Contrast 
267 
have simultaneous contrast, at least not by itself; but we have here 
also successive contrast, and the phenomena are entirely, or in large 
part, identical with those described in the preceding chapter. In order 
to have simultaneous contrast alone, special pains must be taken to 
keep the fixation of the eye absolutely steady during the experiment. 
Later we shall examine more carefully the phenomena of pure 
simultaneous contrast which continue during steady fixation of the 
eye. Now the phenomena will be described that belong partly to 
simultaneous, but mainly to successive contrast, as they are mani¬ 
fested under ordinary natural conditions of vision. The colour changes 
that occur in these circumstances are exactly the same as those already 
described for pure successive contrast. In general they are much more 
distinct and striking than those of pure simultaneous contrast; and 
when the two might cause different results, those of successive contrast 
invariably predominate in the natural use of the eye; and when both 
evoke the same effects, the alterations of colour always become much 
more considerable when the gaze ceases to be steady and begins to 
wander. 
In general, contrast effects are promoted when the inducing 
colour is more intense than the reacting one, because then the after¬ 
images of the former are more vivid and more lasting. For example, 
if a small wafer of white paper is laid on a coloured sheet, this white 
will have the complementary colour. The colouring is more impressive, 
however, when grey is used instead of white; or even black, since in 
these subjective experiments all black is to be considered as a dark 
grey. However, as a rule, a medium grey is more satisfactory for the 
experiment than black. In such cases the contrast action may go so far 
that a tolerably vivid colour is reversed into the complementary. For 
example, if a small piece of orange-red paper (coloured with red lead) 
is laid on a red glass disc and held up against the bright sky, the reddish 
paper looks a vivid green-blue, that is, complementary to the colour 
of the red glass, being almost its own complementary colour too. 
Moreover, it is conducive to have the inducing colour occupy a 
large part of the visual field, because then the various regions of the 
retina will be frequently and continuously stimulated by this colour 
and fatigued by it. The result is that the contrast colours are particu¬ 
larly vivid when the reacting colour occupies a small field surrounded 
by an extensive ground filled with the inducing colour. In this case, 
it is chiefly simply the colour of the small field that is altered, not that 
of the large field. But the contrast effects are not absent even when 
the two fields are of the same size; the influence then being a mutual 
one, and the colour of each being changed by that of the other.
        

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