§ 14- The Visual Qualities 61
increasingly reddish, as we travel to the right-hand end of
the spectrum. Here is a fourth line of qualities, but a line
which in the spectral series is left incomplete at violet If
we continue it, by adding the purples and carmines, we are
finally brought back to our starting-point, — the red of the
extreme left We notice, however, that this red is not, in
reality, the starting-point of a psychological colour-line; it
is not a pure red, but an orange-red ; the red that stands
at the beginning of the red-yellow line lies outside the
spectrum, toward the carmines.
All the colours that can be distinguished upon these four
lines are ultimate qualities of visual sensation. We speak,
it is true, of pure red and of orange-red ; but these terms
merely indicate the position of the qualities upon a colour¬
line : pure red comes at the beginning, orange-red towards
the middle of the line. No orange-red can be analysed, by
introspection, into a red and an orange. The lines them¬
selves, and with them the system of colour qualities, are
most simply arranged in the form of a square, with Rt K,
G and B at the four corners.
So far, then, we have a single straight line to represent
the sensations of light, and four straight lines, forming the
closed figure of a square, to represent the sensations of
colour. But so far, also, we have dealt with sensations of
colour only under one aspect, — that of colour-tone or hue.
Colours, as was said above, differ further from one another
as darker and lighter. Thus, in the spectrum, yellow is
undoubtedly the lightest, and violet the darkest colour.
Here, then, is a second attribute of colour, the attribute of
light-tone or tint, in virtue of which a sensation of colour
may be matched or equated with a sensation of light. Let
us assume that all the hues upon the lines of our colour*