ILLUSIONS CAUSED BY LIGHT ITSELF.
When playing about the Christmas fire, children
frequently amuse themselves by whirling round and
round a piece of wood, one end of which they have
previously lighted and blown out. In proportion as
the movement becomes more rapid, the path of the red-hot
end becomes more and more connected, until at last
a burning ring is formed, in every part of which the
shining charcoal appears to be at the same time. The
only way of accounting for this illusion is by supposing
that the image formed by the burning stick upon the
retina remains there for an appreciable period, the impression
made by it at one part of its journey remaining
until it returns to its former position. The power
possessed by the retina of retaining impressions
explains a large number of illusions of the same kind.
The chord of a musical instrument, for instance, when
struck, appears to occupy a longer space during the
time it vibrates, than when it is at rest. A rapidly revolving
wheel appears almost solid on account of the
combined images of the spokes seeming to unite into
one homogeneous mass.
The persistence of luminous impressions upon the retina has given rise to the invention of a number of well-known optical toys, amongst which may be mentioned the phenakistiscope, the thaumatrope, the phantascope, and many others.
