From: joe@montebello.soest.hawaii.edu (Joe Dellinger)
Subject: Re: The color violet?
Reply-To: joe@montebello.soest.hawaii.edu
Organization: School of Ocean and Earth Science Technology
Date: Fri, 2 Oct 1992 04:56:57 GMT

In article <1992Sep29.193034.1@stsci.edu>, zellner@stsci.edu writes:
|> If only the blue cones are stimulated, how can we see any color but blue?
|> While our red cones peak at wavelengths shorter than 6000A, we can see
|> some light at 7000A or longer.  But we don't have a word for a color
|> redder than red.  Why do we have the color "violet" that is bluer than
|> blue? 

	I was waiting for somebody with an authoritative reference to jump
in and answer this one, but I think I remember the basic answer. If someone
knows better feel free to point out my error.... this is partly remembered
from a Scientific American article of a few years ago and partly from lots
of other random sources I have read over the years.

*begin lecture number 27938*
	The sensitivity curves for the three kinds of receptors in "average"
persons are schematically like this:

             rrrrrrrrrgrgrgrggggg             bbbb                             
            r        g      r    gggggg      b    bb                           
           r        g        rr        ggg  b       bbb                        
         rr        g           rr         gg           bbbb                    
       rr         g              rr       b ggg           rrbbbb               
    rrr          g                 rr    b     ggg      rr  rrrrrbb            
rbgrbgbgbgbgbgbgbbbbbbbbbbbbbbbbbbbbbrbrbrrrrrrrrrgrrgrggggggggggrgbrbrg       
       Red   Orange   Yellow         Green       Blue    Violet


	The basic idea is the Red and Green cones are much more
similar to each other than either is to the Blue cones, and the
Red cone has a blip of sensitivity out in the violet which is why
Red+Blue = Violet (Magenta really). I don't know about you, but when
I look at a rainbow I see lots of violet at one end and have difficulty
finding the hint of blue wedged between the much wider bands of green
and violet!

	In general there is much more random variation in the Red and Green
receptor genes in humans than there is for the Blue. Most mammals in fact
only have TWO different sorts of color receptors, "RedGreen" and Blue.
In the higher primates the single RedGreen gene has become duplicated and
the two descendant genes are still in the process of evolving apart. Evidence
for this is the greater similarity of the pigments and the fact that the
Red and Green pigment genes are also very similar and lie right next to
each other near one end of the X chromosome, an unstable configuration that
makes it much more likely for them to mutate or recombine incorrectly. In fact,
because of the high rate of recombination errors a significant fraction of
the population has variant numbers of copies of one or both of these genes
on one or both of their X chromosomes. (Something like 1 out of 5 people,
as I recall... The similarity and nearness of the Red and Green genes makes
it easy for them to accidentally get parts swapped between them incorrectly,
which can result in mangling one or both of the genes or even sending 3 genes
off one way and 1 the other instead of 2 and 2!) People with extra copies are
usually OK, perhaps with slightly variant color spectral sensitivity. Those
_missing_ one or the other gene completely have "Red Green color blindness".
Their vision is similar to how non-primate mammals must see. Women have two
X's, so even if one X is completely lacking a gene the other can take up the
slack. Males have no such backup, and this combined with the high rate of
errors in the population is why about 1 out of 20 males are significantly
colorblind.

	The "Blue" gene lies in a safe part of Chromosome 4 and displays
much less variation across the population. People lacking the ability
to see Blue as a color because of defective Blue genes (groan) exist but
it's a much rarer condition than Red-Green color blindness.

	All three of the Red, Green, and Blue pigment genes are evolved
descendants of the grey-scale color system more primitive animals first
evolved. Some people are known who have no working copies of THAT gene,
but this is also rare and somewhat serious: In low light the cones cut out
as usual, and the nonfunctional rods mean they are left night blind.
Other species are ahead of us: while we are still tinkering with 3-Color
vision many birds already have FOUR-color vision down, and some species
such as ducks are now working on FIVE.

	Moral: If you want to still to have your artwork admired in 100,000
years, stick to black and white! If evolution continues as it has the color
stuff just isn't going to look the same to our descendants as it does to us
primitives.

*lecture 27938 completed*

     /\    /\    /\/\/\/\/\/\/\.-.-.-.-.......___________
    /  \  /  \  /Hawaii Institute of Geophysics, Honolulu\/\/\.-.-....__
___/    \/    \/Joe Dellinger, Internet: joe@montebello.soest.hawaii.edu\/\.-.__
____________________________________________________
Objectivity is not an unobtainable emptying of mind |
but a willingness to abandon a set of preferences   |----> Stephen J. Gould
when the world seems to work in a contrary way.     |