[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 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.  Next make a dent with a lead pencil at the exact centre on the vane side, and revolve the pencil until the dent is well polished.

[Illustration:  Fig. 146.]

Hold a pin, point upwards, in the right hand, and with the left centre the mill, vanes pointing downwards, on the pin (Fig. 146).  The mill will immediately commence to revolve at a steady pace, and will continue to do so indefinitely; though, if the head of the pin be stuck in, say, a piece of bread, no motion will occur.  The secret is that the heat of the hand causes a very slight upward current of warmed air, which is sufficient to make the very delicately poised windmill revolve.