direction of the white light striking the prism,
are said to be the most refrangible rays, and consequently are situated
in the most refrangible end or part of the spectrum, namely, that
farthest from the original direction of the incident white light. These
most refrangible rays are the violet, and we pass on to the least
refrangible end, the red, through bluish-violet, blue, bluish-green,
green, greenish-yellow, yellow, and orange. If you placed a prism say in
the red part of the spectrum, and caught some of those red rays and
allowed them to pass through your prism, and then either looked at the
emerging light or let it fall on a white surface, you would find only
red light would come through, only red rays. That light has been once
analysed, and it cannot be further broken up. There is great diversity
of shades, but only a limited number of primary impressions. Of these
primary impressions there are only four--red, yellow, green, and blue,
together with white and black. White is a collective effect, whilst
black is the antithesis of white and the very negation of colour. The
first four are called primary colours, for no human eye ever detected in
them two different colours, while all of the other colours contain two
or more primary colours. If we mix the following tints of the spectrum,
_i.e._ the following rays of coloured light, we shall produce white
light, red and greenish-yellow, orange and Prussian blue, yellow and
indigo blue, greenish-yellow and violet. All those pairs of colours that
unite to produce white are termed complementary colours. That is, one is
complementary to the other. Thus if in white light you suppress any one
coloured strip of rays, which, mingled uniformly with all the rest of
the spectral rays, produces the white light, then that light no longer
remains white, but is tinged with some particular tint. Whatever colour
is thus suppressed, a particular other tint then pervades the residual
light, and tinges it. That tint which thus makes its appearance is the
one which, with the colour that was suppressed, gave white light, and
the one is complementary to the other. Thus white can always be
compounded of two tints, and these two tints are complementary colours.
But it is important to remark here that I am now speaking of rays of
coloured light proceeding to and striking the eye; for a question like
this might be asked: "You say that blue and yellow are complementary
colours, and together they p