Cut out two card-board cog-wheels of equal size; place them upon a
pin, and whirl them round with equal velocity in opposite directions;
when, instead of producing a hazy tint, as one wheel would do, or as
the two would if revolving in the same direction, there will be an
extraordinary appearance of a fixed wheel. If the cogs be cut slantwise
on both wheels, the spectral wheel, as it may be called, will exhibit
slanting cogs; but if one of the wheels be turned, so that the cogs
shall point in opposite directions, then the spectral wheel will have
straight cogs. If wheels with radii, or arms, be viewed when moving,
the deception will be similar; and however fast the wheels may move,
provided it be with equal velocity, the magic of a fixed wheel will be
presented.
Or, cut a card-board wheel with a certain number of teeth or cogs
at its edge; a little nearer the centre, cut a series of apertures
resembling the cogs in arrangement, but not to the same number; and
still nearer the centre cut another series of apertures, different in
number, and varying from the former. Fix this wheel upon another, with
its face held two or three yards from an illuminated mirror; spin it
round, the cogs will disappear, and a greyish belt, three inches broad,
will become visible; but, on looking at the glass through the moving
wheel, appearances will entirely change; one row of cogs, or apertures,
will appear fixed, as if the wheel were not moving, whilst the other
two will appear as if in motion; and, by shifting the eye, other and
new effects appear.
These amusing deceptions were first experimented by Mr. Faraday. The
simple apparatus for their exhibition may be purchased, for a trifling
sum, of any respectable optician.