THEORIES OF COLOR VISION
The statements I have made 
about the relationship between 
what cones are stimulated and 
what we see depend on research 
that began with Newton in 1704 
and continues up to the 
present. The ingenuity of 
Newton's experiments is hard to 
exaggerate: in his work on 
color, he split up white light 
with a prism; he recombined 
the light with a second prism, 
obtaining white again; he made 
a top consisting of colored 
segments, which when spun 
gave white. These discoveries 
led to the recognition that 
ordinary light is made up of a 
continuous mixture of light of 
different wavelengths.

    Gradually, over the 
eighteenth century, it came to 
be realized that any color could 
be obtained by mixtures of light 
of three wavelengths in the 
right proportions, provided the 
wavelengths were far enough 
apart. The idea that any color 
could be produced by 
manipulating three controls (in 
this case, controls of the 
intensity of the three lights) 
was termed trichromacy. In 
1802 Thomas Young put forward 
a clear and simple theory to 
explain trichromacy: he 
proposed that at each point in 
the retina there must exist at 
least three "particles"--tiny 
light-sensitive structures--
sensitive to three colors, red, 
green, and violet. The long time 
span between Newton and 
Young is hard to explain, but 
various roadblocks, such as 
yellow and blue paints mixing 
to produce green, must surely 
have impeded clear thinking. 
The definitive experiments that 
finally proved Young's idea that 
color must depend on a retinal 
mosaic of three kinds of 
detectors was finally confirmed 
directly and conclusively in 
1959, when two groups, George 
Wald and Paul Brown at Harvard 
and William Marks, William 
Dobelle, and Edward MacNichol 
at Johns Hopkins, examined 
microscopically the abilities of 
single cones to absorb light of 
different wavelengths and 
found three, and only three, 
cone types. Meanwhile, 
scientists had had to do the best 
they could by less direct means, 
and they had, in fact, in the 
course of several centuries 
arrived at substantially the 
same result, proving Young's 
theory that just three types of 
cones were necessary and 
estimating their spectral 
sensitivities. The methods were 
mainly psychophysical: 
scientists learned what colors 
are produced with various 
mixtures of monochromatic 
lights, they studied the effects 
on color vision of selective 
bleaching with monochromatic 
lights, and they studied color 
blindness.