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
206, 207.] 
§23. Variations of Sensitivity 
excessively, it also excites the others a little; and if, owing to the strong 
stimulation, the sensitivity of the former fibres is more rapidly lowered 
than that of the latter, the colour impression must approach pale or 
grey red. 
When the primary colour is complementary to the reacting colour 
of the ground, the latter colour comes out more saturated in the exten¬ 
sion of the after-image than it does on the parts of the retina that 
have not been fatigued or on the parts that have been fatigued by the 
colour of the ground. If a blue-green object is placed on a red ground, 
and then removed after having been steadily fixated, a saturated red 
after-image appears, just as if a black object had been removed. But 
it is easy to realize that the colour in the after-image of a comple¬ 
mentary object is even more saturated than in the after-image of a 
black object. The simplest way. of doing it is to prepare an object 
which is part black and part coloured, for example, blue-green. Now 
place it on a complementary (red) ground and look at a point on the 
ground close to the border between black and blue-green; and then 
take the object away, and all over the after-image the colour of the 
ground will come out clearer than it was previously in the part of the 
ground that was uncovered. The after-image of the blue-green is 
somewhat darker than that of the black, but it is not as if the red were 
less luminous there; it is more as if the red in the after-image of the 
black were overspread by a pale mist, which does not interfere with 
the red in the after-image of the blue-green. Thus the after-image of 
red on red looks grey-red; of black on red white-red; and of blue-green 
on red, saturated red. These distinctions come out very plainly when 
the experiment is performed with all three shades side by side. 
On the supposition that there is some white in the red of the 
ground, the result is easily explained. The eye is not at all fatigued by 
black. Its sensitiveness to the whitish red in the after-image of the 
ground is unaltered. The eye is fatigued for red by red. Its sensitiveness 
to red in the after-image is lowered, but its sensitiveness to the other 
constituents of white is practically undiminished, the sensation being 
that of faint whitish red (grey-red). On the other hand, blue-green 
renders the eye less sensitive to the non-red constituents of the ground 
light, and therefore leaves the red to stand out in the after-image with 
less foreign admixtures. 
Similar experiments are just as successful with pure spectral 
colours. In the field of a telescope the author has projected separate 
portions of the spectrum, taking every possible precaution to get rid 
of the last traces of white light. The ground was so densely black that 
the outline of the diaphragm of the telescope could not even be 
distinguished on it, and all that could be seen were the cloudy patterns


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