The next thing is to get some 
feel for what it means for our 
color vision to have three 
visual receptors. First, you 
might ask, if a given cone works 
better at some wavelengths 
than at others, why not simply 
measure that cone's output and 
deduce what the color is? Why 
not have one cone type, instead 
of three? It is easy to see why. 
With one cone, say the red, you 
wouldn't be able to tell the 
difference between light at the 
most effective wavelength, 
about 560 nanometers, from a 
brighter light at a less effective 
wavelength. You need to be able 
to distinguish variations in 
brightness from variations in 
wavelength.

    But suppose you have two 
kinds of cones, with 
overlapping spectral 
sensitivities--say, the red cone 
and the green cone. Now you 
can determine wavelength 
simply by comparing the 
outputs of the cones. For short 
wavelengths, the green cone 
will fire better; at longer and 
longer wavelengths, the 
outputs will become closer and 
closer to equal; at about 580 
nanometers the red surpasses 
the green, and does 
progressively better relative to 
it as wavelengths get still 
longer. If we subtract the 
sensitivity curves of the two 
cones (they are logarithmic 
curves, so we are really taking 
quotients), we get a curve that 
is independent of intensity. So 
the two cones together now 
constitute a device that 
measures wavelength.

    Then why are not two 
receptors all we need to 
account for the color vision 
that we have? Two would indeed 
be enough if all we were 
concerned with was 
monochromatic light--if we 
were willing to give up such 
things as our ability to 
discriminate colored light from 
white light. Our vision is such 
that no monochromatic light, 
at any wavelength, looks white. 
That could not be true if we had 
only two cone types. In the case 
of red and green cones, by 
progressing from short to long 
wavelengths, we go 
continuously from stimulating 
just the green cone to 
stimulating just the red, 
through all possible green-to-
red response ratios. White light, 
consisting as it does of a 
mixture of all wavelengths, has 
to stimulate the two cones in 
some ratio. Whatever 
monochromatic wavelength 
happens to give that same ratio 
will thus be indistinguishable 
from white. This is exactly the 
situation in a common kind of 
color blindness in which the 
person has only two kinds of 
cones: regardless of which one 
of the three pigments is missing 
there is always some 
wavelength of light that the 
person cannot distinguish from 
white. (Such subjects are color 
defective, but certainly not 
color-blind.)