appear not to move, but gradually to fade away.

This phenomenon was first observed by Münsterberg, and by him shown to
Jastrow,[1] who, with Moorehouse, has printed a study, without,
however, offering an adequate explanation of it.

[1] Jastrow, J., and Moorehouse, G.W.: 'A Novel Optical
Illusion,' _Amer. Jour. of Psychology_, 1891, IV., p. 201.


I. APPARATUS FOR PRODUCING THE ILLUSION.


Any form of color-wheel may be used, but preferably one which is
driven by electricity or clock-work, so that a fairly constant speed
is assured. Several pairs of paper discs are needed, of the ordinary
interpenetrating kind which permit a ready readjustment of the ratios
between the two sectors, as follows: one pair consisting of a white
and a black disc, one of a light-and a dark-colored disc (light green
and dark red have been found admirably suited to the purpose), and a
pair of discs distinctly different in color, but equal in luminosity.

The rod should be black and not more than a quarter of an inch broad.
It may be passed before the rotating disc by hand. For the sake of
more careful study, however, the rod should be moved at a constant
rate by some mechanical device, such as the pendulum and works of a
Maelzel metronome removed from their case. The pendulum is fixed just
in front of the color-disc. A further commendable simplification of
the conditions consists in arranging the pendulum and disc to move
concentrically, and attaching to the pendulum an isosceles-triangular