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
§24. Contrast 
side by the sun, so that no light goes directly into the pupil, the other 
eye meantime being shaded, white objects will look greenish to the 
right eye and reddish to the left eye. This is seen distinctly by opening 
the two eyes in succession, sometimes the right eye and sometimes the 
left eye ; or by looking steadily with both eyes at a white sheet of paper 
and holding a little black rod vertically midway between the paper and 
the eyes. Then two images of the rod will be seen projected on the 
paper, one for each eye. The image on the left, where the surface of the 
paper is seen by the left eye, but not by the right eye, will look red, and 
the other image will look green. On the other hand, when a person 
looks steadily at a black plate and holds a white object in front of it 
some distance away, so that there are two images of it, the right image, 
which now is the one seen by the left eye, will be red, and the left image 
will be green. Thus, white looks greener to the eye that is illuminated 
from one side than it does to the eye that is not illuminated. Now 
under these circumstances, light penetrates through the sclerotica and 
eyelids into the illuminated eye, and this light is red, as we already 
know from previous experiments (Vol. I, p. 213). If sunlight is allowed 
to shine on the eye from one side, the red colour will be recognized on 
dark objects too. For example, on looking at a printed page, the black 
letters appear a beautiful red and the white paper green. This red 
light coming in from the side is diffused over most of the fundus of the 
eye, and the places on the retina of the illuminated eye where the image 
of a white object is formed are therefore simultaneously illuminated 
by white and red light, but the sensation is greenish white. The green¬ 
ish colouring gets more and more distinct as the experiment goes on, 
because it depends on the eye’s being fatigued for red. But with ex¬ 
cessive red illumination of the retina the only way this can happen 
is by the illumination already diffused over the ground getting sep¬ 
arated from the additional light coming from the objects; and thus 
this latter light looks greenish because the eye is fatigued for red. In 
contrast therewith pure white looks reddish in the eye that has not 
been affected. 
Consider, moreover, the image of the wall-paper and of the ceiling 
of a room which is reflected in the highly polished surface of the top of a 
mahogany table. When the eye is accommodated for these images, 
the colours either look natural or, it may be, a little bluish, comple¬ 
mentary to the colour of the table. On the other hand, when the 
eye is accommodated for the top of the table, the total light coming 
from it is overwhelmingly red-yellow. The author’s experience in 
this case is that the complementary colouring of the images occurs 
especially when the reflected light of the object is feeble as compared


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