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
eighth of an inch away draw dotted parallel lines, all on the same side of
their fellow lines in order of rotation. Cut out along the large circle,
and then with a. sharp knife follow the lines shown double in Fig. 145.
This gives eight little vanes, each of which must be bent upwards to
approximately the same angle round a flat ruler held with an edge on the
dotted line.
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