LECTURE   VI

§ I. Light-sensations ; Sensations of Colour and Brightness ;
Analysis and Mixture of Colours. § II. The Three Funda-
jhental Colours. § III. Leonardo's Four Principal Colours.
§ IV. Theory of Light-sensations.

§ I

VISUAL sensations have two qualities,—colour and bright¬
ness. The latter term includes black, white, and all the
various shades of grey. The number of distinguishable colours
is enormous : it has never been determined. But it is a fact
of immediate perception that the variety of colour in nature is
not altogether heterogeneous : there are very many intermediate
tints between this colour and that. And if we try to divide up
the whole multifarious colour field, and to separate out the colours
which are clearly and definitely unlike the rest, we can reduce the
list of'pure' colours to very small dimensions. Red, yellow,
green, blue, together with black and white, prove to be the
simple and ultimate qualities which we are able, so to say, to
abstract from the innumerable specifically different presenta¬
tions of nature. All other distinguishable colour-tones are
intermediates,—a fact which is very frequently expressed in the
names given to them (purplish red, orange-yellow, yellowish
green, violet-blue, etc.). But these six simple qualities, again,
are not co-ordinates ; they evince different degrees of variety or
resemblance. We are inclined to regard green as being nearer
blue than yellow is, and to look on red and yellow as pretty
closely related colours, even when the intermediates blue-green
and orange are not present to suggest the comparison. It might
be thought that this notion of colour relationship was due to our
knowledge of colour sequence in the rainbow. But children
who have never observed a rainbow with any degree of attention
will usually connect blue with green, and red with yellow, when
required to arrange the four qualities in the order of likeness.