That color can be so 
computed predicts color 
constancy because what counts 
for each projector are the ratios 
of light from one region to light 
from the average surround. The 
exact intensity settings of the 
projectors no longer matter: the 
only stipulation is that we have 
to have some light from each 
projector; otherwise no ratio 
can be taken. One consequence 
of all this is that to have color 
at all, we need to have 
variation in the wavelength 
content of light across the 
visual field. We require color 
borders for color, just as we 
require luminance borders for 
black and white. You can easily 
satisfy yourself that this is 
true, again using two slide 
projectors. With a red filter (red 
cellophane works well) in front 
of one of the projectors, 
illuminate any set of objects. My 
favorite is a white or yellow 
shirt and a bright red tie. When 
so lit, neither the shirt nor the 
tie looks convincingly red: both 
look pinkish and washed out. 
Now you illuminate the same 
combination with the second 
projector, which is covered 
with blue cellophane. The shirt 
looks a washed-out, sickly 
blue, and the tie looks black: 
it's a red tie, and red objects 
don't reflect short wavelengths. 
Go back to the red projector, 
confirming that with it alone, 
the tie doesn't look especially 
red. Now add in the blue one. 
You know that in adding the 
blue light, you will not get 
anything more back from the 
tie--you have just demonstrated 
that--but when you turn on the 
blue projector, the red tie 
suddenly blazes forth with a 
good bright red. This will 
convince you that what makes 
the tie red is not just the light 
coming to you from the tie.

    Experiments with stabilized 
color borders are consistent 
with the idea that differences 
across borders are necessary for 
color to be seen at all. Alfred 
Yarbus, whose name came up in 
the context of eye movements 
in Chapter 4, showed in 1962 
that if you look at a blue patch 
surrounded by a red 
background, stabilizing the 
border of the patch on the 
retina will cause it to 
disappear: the blue melts away, 
and all you see is the red 
background. Stabilizing the 
borders on the retina 
apparently renders them 
ineffective, and without them, 
we have no color.

    These psychophysical 
demonstrations that 
differences in the spectral 
content of light across the 
visual field are necessary to 
perceive color suggest that in 
our retinas or brains we should 
find cells sensitive to such 
borders. The argument is 
similar to the one we made in 
Chapter 4, about the perception 
of black or white objects (such 
as kidney beans). If at some 
stage in our visual path color is 
signaled entirely at color-
contrast borders, cells whose 
receptive fields are entirely 
within areas of uniform color 
will be idle. The result is 
economy in handling the 
information. We thus find 
ourselves with two advantages 
to having color signaled at 
borders: first, color is 
unchanged despite changes in 
the light source, so that our 
vision tells us about properties 
of the objects we view, 
uncontaminated by 
information about the light 
source; second, the 
information handling is 
economical. Now we can ask 
why the system evolved the way 
it did. Are we to argue that the 
need for color constancy led to 
the system's evolving and that 
an unexpected bonus was the 
economy--or the reverse, that 
economy was paramount and 
the color constancy a bonus? 
Some would argue that the 
economy argument is more 
compelling: evolution can 
hardly have anticipated 
tungsten or fluorescent lights, 
and until the advent of 
supersuds, our shirts were not 
all that white anyway.