245.]
§24. Contrast
289
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