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d and e are measurements of normalized light intensity, then the
chromaticity of the unknown spectral light f( S ) may be calculated.
Thus when for instance:
a ( R ) + b ( G ) + c ( B ) = W = d ( R ) + e ( G ) + f( S ), then
a ( R ) + b ( G ) + c ( B ) = d ( R ) + e ( G ) + f ( S ) and
f( S ) = ( a - d )( R ) + ( b - e )( G ) + c ( B )
(It should be noted that the equals sign, '=', reads; 'is matched by'.)
As will be seen, the application of Newton's law of gravity made
allowance for an algebraic treatment of the colour-mixture data and,
hence, for the use of negative values of standard lights. The problem
pointed out by Helmholtz ( 1852 ), that monochromatic spectral lights
are too saturated to be matched by a mixture of primaries, could
then be resolved on the assumption that each of the three receptors
is activated by a large part of the spectrum. The mixture of two
or three primaries may, therefore, activate the white component
mechanism more than monochromatic spectral lights and, hence,
be less saturated. In accord with this assumption, Maxwell ( 1860 )
showed that by adding one of the three standard lights to the spectral
test light it could be desaturated (i.e. the white component of the
spectral test light could be increased) to such an extent that it could
be matched by the mixture of the two other standard lights.
The evidence provided by Maxwell ( 1855 , 1860 ) was generally
taken as conclusive evidence in favour of the basic assumption that
the central fovea contains three different types of receptor. Thus, it was
argued that if colours were determined by the relative activity of three
independent receptor processes in the retina, then, by the use of three
standard stimuli, one should be capable of stimulating the receptor
systems in all possible combinations and, thereby, reproducing any of
the spectral colours as demonstrated by Maxwell ( 1855 , 1860 ).
Maxwell adduced further strong evidence in favour of the
triplicity of the normal retina by demonstrating that a protanope
('red'-colour-blind) could match every colour in the spectrum by using
only two primaries. In fact, his results suggested that the protanope
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