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.
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