be
compounded into white. H. von Helmholtz has shown that the only pair of
simple spectral colours capable of compounding to white are a
greenish-yellow and blue.

[Illustration: FIG. 1.]

Just as musical sounds differ in pitch, loudness and quality, so may
colours differ in three respects, which Maxwell calls _hue_, _shade_ and
_tint_. All hues can be produced by combining every pair of primaries in
every proportion. The addition of white alters the tint without
affecting the hue. If the colour be darkened by adding black or by
diminishing the illumination, a variation in shade is produced. Thus the
hue red includes every variation in tint from red to white, and every
variation in shade from red to black, and similarly for other hues. We
can represent every hue and tint on a diagram in a manner proposed by
Young, following a very similar suggestion of Newton's. Let RGB (fig. 1)
be an equilateral triangle, and let the angular points be coloured red,
green and blue of such intensities as to produce white if equally
combined; and let the colour of every point of the triangle be
determined by combining such proportions of the three primaries, that
three weights in the same proportion would have their centre of gravity
at the point. Then the centre of the triangle will be a neutral tint,
white or grey; and the middle points of the sides Y, S, P will be
yellow, greenish-blue and purple. The hue varies all round the
perimeter. The tint varies along any straight line through W. To vary
the shade, the whole triangle must be uniformly darkened.

The simplest way of compounding colours is by means of Maxwell's colour
top, which is a broad spinning-top over the spindle of which coloured
disks can be slipped (fig. 2). The disks are slit radially so that they
can be slipped partially over each other and the surfaces exposed in any
desired ratio. Three disks are used together, and a match is obtained
between these and a pair of smaller ones mounted on the same spindle. If
any five colours are taken, two of which may be black and white, a match
can be got between them by suitable adjustment. This shows that a
relation exists between any four colours (the black being only needed to
obtain the proper intensity) and that consequently the number of
independent colours is three. A still better instrument for combining
colours is Maxwell's colour box, in which the colours of the spectrum
are combined by means of prisms. Sir W. Abney has