some light may be absorbed and some reflected. For many objects,
the relative amount of light absorbed and reflected depends on the light's wavelength. The green leaf of a plant absorbs long- and short-wavelength light and reflects light of middle wavelengths, so that when the sun hits
the light reflected back will have a pronounced broad peak at middle wavelengths (in the green). A red object will have its peak, likewise broad,
in the long wavelengths, as shown in the graph on this page.
An object that absorbs some of the light reaching it and reflects the
rest is called a pigment. If some wavelengths in the range of visible light are
absorbed more than others, the pigment appears to us to be colored. What color
we see, I should quickly add, is not simply a matter of wavelengths; it depends
on wavelength content and on the properties of our visual system. It involves both physics and biology.
Each rod or cone in our retina contains a pigment that absorbs some wavelengths better than others. The pigments, if we were able to
get enough of them to look at, would therefore be colored. A visual pigment
has the special property that when it absorbs a photon of light, it changes
its molecular shape and at the same time releases energy. The release sets
off a chain of chemical events in the cell, described in Chapter 3, leading
ultimately to an electrical signal and secretion of chemical transmitter at the
synapse. The pigment molecule in its new shape will generally have quite different
lightabsorbing properties, and if, as is usually the case, it absorbs light
less well than it did before the light hit it, we say it is bleached by the light.
A complex chemical machinery in the eye then restores the pigment to its original
conformation; otherwise, we would soon run out of pigment.
Our retinas contain a mosaic of four types of receptors: rods and three
types of cones, as shown in the illustration at the top of the facing page.
Each of these four kinds of receptors contains a different pigment. The pigments
differ slightly in their chemistry and consequently in their relative ability
to absorb light of different wavelengths. Rods are responsible for our ability
to see in dim light, a kind of vision that is relatively crude and completely
Rod pigment, or rhodopsin, has a peak sensitivity at about 510 nanometers,
in the green part of the spectrum. Rods differ from cones in many ways:
they are smaller and have a somewhat different structure; they differ from cones
in their relative numbers in different parts of the retina and in the connections they make with subsequent stages in the visual pathway. And finally,
in the light-sensitive pigments they contain, the three types of cones themselves
differ from each other and from rods.
The pigments in the three cone types have their peak absorptions at
about 430, 530, and 560 nanometers, as shown in the graph on this page; the
cones are consequently loosely called "blue", "green",
and "red", "loosely" because (i) 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 ofjust 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,
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
Thus the red-sensitive cone does not respond only to long-wavelength,
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 threecentury history of how these facts were established or how the brain
COMMENTS ON COLOR
It may be useful to begin by comparing the way our auditory systems and our visual systems deal with wavelength. One system leads to
tone and the other to color, but the two are profoundly different. When I
play a chord of five notes on the piano, you can easily pick out the individual
notes and sing them back to me. The notes don't combine in our brain but preserve their individuality, whereas since Newton we have known that if you
mix two or more beams of light of different colors, you cannot say what the
components are, just by looking.
A little thought will convince you that color vision has to be an impoverished sense, compared with tone perception. Sound coming to one ear
at any instant, consisting of some combination of wavelengths, will influence
thousands of receptors in the inner ear, each tuned to a slightly different
pitch than the next receptor. If the sound consists of many wavelength components,
the information will affect many receptors, all of whose outputs are sent
to our brains. The richness of auditory information comes from the brain's
ability to analyze such combinations of sounds.