[Illustration: FIG. 142.]
[Illustration: FIG. 143.]
The Magic Spokes (Fig. 143).--Place a design like this on the gramophone
and let it turn at high speed. The radial lines seem but a blur. Now punch
a hole one-eighth of an inch in diameter in a piece of blackened card, and,
standing well away from the gramophone, apply your eye to the hole and move
the card quickly to and fro. The extreme briefness of the glimpses obtained
of the moving lines seems to rob them of motion, or even make them appear
to be moving in the direction contrary to the actual. Instead of a single
hole, one may use a number of holes punched at equal intervals round a
circle, and revolve the card on the centre. If a certain speed be
maintained, the spokes will appear motionless.
The substitution of a long narrow slit for a circular hole gives other
effects.
[Illustration: FIG. 144.]
A Colour Top.--Cut a 4-inch disc out of white cardboard and blacken
one-half with Indian ink. On the other half draw four series of concentric
black lines, as shown in Fig. 144. If the disc is mounted on a knitting
needle and spun in a horizontal plane, the black lines will appear of
different colours.
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