Absorption spectra (or 
sensitivity curves) differ for the 
three types of cones. (Spectral-
energy curves and absorption 
curves such as these have their 
y axes in log units because they 
operate over such a wide range. 
The up-and-down position of 
the x-axis is therefore 
arbitrary and does not 
represent zero absorption.)

    The pigments in the three 
cone types have their peak 
absorptions at about 430, 530, 
and 560 nanometers, as shown 
in the graph to the left; the 
cones are consequently loosely 
called "blue", "green", and 
"red", "loosely" because (1) 
the names refer to peak 
sensitivities (which in turn are 
related to ability to absorb light) 
rather than to the way the 
pigments would appear if we 
were to look at them; (2) 
monochromatic lights whose 
wavelengths are 430, 530, and 
560 nanometers are not blue, 
green, and red but violet, blue-
green, and yellow-green; and 
(3) if we were to stimulate cones 
of just one type, we would see 
not blue, green, or red but 
probably violet, green, and 
yellowish-red instead. However 
unfortunate the terminology is, 
it is now widely used, and 
efforts to change embedded 
terminology usually fail. To 
substitute terms such as long, 
middle, and short would be 
more correct but would put a 
burden on those of us not 
thoroughly familiar with the 
spectrum.

    With peak absorption in the 
green, the rod pigment, 
rhodopsin, reflects blue and red 
and therefore looks purple. 
Because it is present in large 
enough amounts in our retinas 
that chemists can extract it and 
look at it, it long ago came to be 
called visual purple. Illogical as 
it is, "visual purple" is named 
for the appearance of the 
pigment, whereas the terms for 
cones, "red", "green", and 
"blue", refer to their relative 
sensitivities or abilities to 
absorb light. Not to realize this 
can cause great confusion.

    The three cones show broad 
sensitivity curves with much 
overlap, especially the red and 
the green cones. Light at 600 
nanometers will evoke the 
greatest response from red 
cones, those peaking at 560 
nanometers, but will likely 
evoke some response, even if 
weaker, from the other two 
cone types. Thus the red-
sensitive cone does not respond 
only to long-wavelength, or 
red, light; it just responds 
better. The same holds for the 
other two cones.

    So far I have been dealing 
with physical concepts: the 
nature of light and pigments, 
the qualities of the pigments 
that reflect light to our eyes, 
and the qualities of the rod and 
cone pigments that translate 
the incoming light into 
electrical signals. It is the 
brain that interprets these 
initial signals as colors. In 
conveying some feel for the 
subject, I find it easiest to 
outline the elementary facts 
about color vision at the outset, 
leaving aside for the moment 
the three-century history of 
how these facts were 
established or how the brain 
handles color.