enious implement devised
by Clerk-Maxwell, which gives us a result of mixing colors without the
chemical risks of letting them come in contact, and also measures
accurately the quantity of each which is used (Fig. 17).

(107) This is called a Maxwell disc, and is nothing more than a circle
of firm cardboard, pierced with a central hole to fit the spindle of a
rotary motor, and with a radial slit from rim to centre, so that another
disc may be slid over the first to cover any desired fraction of its
surface. Let us paint one of these discs with Venetian red and the other
with viridian and cobalt, the first pair in the list of pigments to be
used on the globe.

(108) Having dried these two discs, one is combined with the other on
the motor shaft so that each color occupies half the circle. As soon as
the motor starts, the two colors are no longer distinguished, and rapid
rotation melts them so perfectly that the eye sees a new color, due to
their mixture on the retina. This new color is a reddish gray, showing
that the red is more chromatic than the blue-green. But by stopping the
motor and sliding the green disc to cover more of the red one, there
comes a point where rotation melts them into a perfectly neutral gray.
No hint of either hue remains, and the pair is said to balance.

(109) Since this balance has been obtained by _unequal areas_ of the two
pigments, it must compensate for a lack of equal chroma in the hues (see
paragraphs 76, 77); and, to measure this inequality, a slightly larger
disc, with decimal divisions on its rim, is placed back of the two
painted ones. If this scale shows the red as occupying 3-1/3 parts of
the area, while blue-green occupies 6-2/3 parts, then the blue-green
must be only half as chromatic as the red, since it takes twice as much
to produce the balance.

(110) The red is then grayed (diminished in chroma by additions of a
middle gray) until it can occupy half the circle, with blue-green on the
remaining half, and still produce neutrality when mixed by rotation.
Each disc now reads 5 on the decimal scale. Lest the graying of red
should have disturbed its value, it is again tested on the photometric
scale, and reads 4.7, showing it has been slightly darkened by the
graying process. A little white is therefore added until its value is
restored to 5.

(111) The two opposites are now completely balanced, for they are equal
in value (5), equal in chroma (5), and have proved their equal