case we do not get a true
combination of the colours at all. When the mixed pigments are
illuminated by white light, the yellow particles absorb the red and blue
rays, but reflect the yellow along with a good deal of the neighbouring
green and orange. The blue particles, on the other hand, absorb the red,
orange and yellow, but reflect the blue and a good deal of green and
violet. As much of the light is affected by several particles, most of
the rays are absorbed except green, which is reflected by both pigments.
Thus, the colour of the mixture is not a mixture of the colours yellow
and blue, but the remainder of white light after the yellow and blue
pigments have absorbed all they can. The effect can also be seen in
coloured solutions. If two equal beams of white light are transmitted
respectively through a yellow solution of potassium bichromate and a
blue solution of copper sulphate in proper thicknesses, they can be
compounded on a screen to an approximately white colour; but a single
beam transmitted through both solutions appears green. Blue and yellow
pigments would produce the effect of white only if very sparsely
distributed. This fact is made use of in laundries, where cobalt blue is
used to correct the yellow colour of linen after washing.

Thomas Young suggested red, green and violet as the primary colours, but
the subsequent experiments of J. Clerk Maxwell appear to show that they
should be red, green and blue. Sir William Abney, however, assigns
somewhat different places in the spectrum to the primary colours, and,
like Young, considers that they should be red, green and violet. All
other hues can be obtained by combining the three primaries in proper
proportions. Yellow is derived from red and green. This can be done by
superposition on a screen or by making a solution which will transmit
only red and green rays. For this purpose Lord Rayleigh recommends a
mixture of solutions of blue litmus and yellow potassium chromate. The
litmus stops the yellow and orange light, while the potassium chromate
stops the blue and violet. Thus only red and green are transmitted, and
the result is a full compound yellow which resembles the simple yellow
of the spectrum in appearance, but is resolved into red and green by a
prism. The brightest yellow pigments are those which give both the pure
and compound yellow. Since red and green produce yellow, and yellow and
blue produce white, it follows that red, green and blue can