lour.

[Illustration: FIG. 141.]

Another optical illusion is afforded by Fig. 142. Two sets of circles are
described about different centres, and the crescent-shaped areas between
them coloured, the remainder of the disc being left white. The disc is
revolved about the centre of the white areas, and one gets the impression
that the coloured parts are portions of separate discs separated by white
discs.

[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. A clockwise rotation makes the outermost lines appear a
greenish blue, those nearest the centre a dark red, and the intermediate
groups yellow and green. A reversal of the motion reverses the order of the
colours, the red lines now being farthest from the centre. The experiment
is generally most successful by artificial light, which contains a larger
proportion of red and yellow rays than does sunlight. The speed at which
the top revolves affects the result considerably. It should be kept
moderate, any excess tending to neutralize the colours.

[Illustration: FIG. 145.]

The Magic Windmill.--Mark a circle 2-1/2 inches in diameter on a piece of
notepaper, resting the centre leg [of the compass] so lightly that it dents
without piercing the paper. With the same centre describe a 3/4-inch
circle. Join the circles by eight equally spaced radial lines, and an