PROVIDE a circle of card-board, six inches in diameter; divide it
into sixteen parts, and paint them alternately red and black. Provide
a second circle or disc of the same size, and paint on it, in large
characters, the words “At rest,” on a white ground. Connect both discs
with the simple apparatus for causing them to turn round, used in the
construction of a toy windmill. Next fill a basin with water, and
provide a few small pieces of phosphuret of lime: darken the room,
hold the discs over the basin, and turn them round; let the phosphuret
of lime be put into the water, and bubbles of light will rise to its
surface. If they come up slowly, both discs will appear stationary
during their turning round; but when the bubbles come up quickly, the
black and red spaces will exhibit a dancing motion, and sometimes
two black spaces will seem joined into one, to the exclusion of the
intervening red, and _vice versâ_: the words on the second disc will
also cross each other in various directions, when the flashes of
light interfere; and, in both cases, confusion will be excited by an
impression being made on the retina, before preceding impressions have
departed.