[Illustration: Fig. 24.]
Saw a tube in half lengthwise, and at one end mount a grooved wheel,
over which passes the bight of a cord, with its two ends passing out of
the hollow cylinder at the sides of the other end. Tint one half of the
cord a different colour from the other, close it in; varnish well to
hide the crack, and your trick is complete.
The cord seems to have the chameleon property of changing its hue.