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 colour.
[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.
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