If the disc is turned in an anti-clockwise direction,
the lines seem to move towards the circumference and disappear. To get the
proper effect the gaze should be fixed and not attempt to follow the lines
round.
[Illustration: FIG. 137.]
[Illustration: FIG. 138.]
The Rolling Circles.--Figs. 138 and 139 are variations of the same idea.
In Fig. 138 two large circles are described cutting one another and
enclosing a smaller circle concentric with the disc. When spun at a certain
rate the larger circles will appear to run independently round the small.
The effect is heightened if the circles are given different colours. If
black only is used for the large circles, the eyes should be kept half
closed. In Fig. 139 two pairs of circles are described about two centres,
neither of which is the centre of the disc. The pairs appear to roll
independently.
[Illustration: FIG. 139.]
[Illustration: FIG. 140.]
The Wriggling Line (Fig. 140).--If this design is revolved at a low speed
and the eye is fixed on a point, the white (or coloured) line will seem to
undulate in a very extraordinary manner.
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