awn should be made as large as the gramophone table will take
conveniently, so as to be viewed by a number of people at once. To
encourage readers who do not possess a gramophone, it may be pointed out
that a gramophone, is merely a convenience, and not indispensable for
turning the discs, which may be revolved on a sharpened pencil or any other
spindle with pointed ends.

The Vanishing Spirals (Fig. 137).--This design, if spun slowly in a
clockwise direction, gives one the impression that the lines all move in
towards the centre. 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. The line is made up of arcs of
circles, and as the marking out is somewhat of a geometrical problem, a
diagram (Fig. 141) is added to show how it is done. The dotted curves are
those parts of the circles which do not enter into the design.

Begin by marking out the big circle A for the disc. The circumference of
this is divided into six equal parts (chord equal to radius), and through
the points of division are drawn the six lines from the centre. Describe
circles aaa, each half the diameter of A. The circles bbb are then drawn
from centres on the lines RRR, and with the same radius as aaa., The same
centres are used for describing the circles a1 a1 a1 and b1 b1 b1, parts of
which form the inner boundary of the line. The background should be
blackened and the belt left white or be painted some bright co