after the removal of a stimulus, that whenever two discontinuous
stimulations follow one another rapidly enough, they will appear
continuous. This fact is a fruitful source of optical illusion. The
appearance of a blending of the stripes of colours on a rotating disc or
top, of the formation of a ring of light by swinging round a piece of
burning wood, and the illusion of the toy known as the thaumatrope, or
wheel of life, all depend on this persistence of retinal impression.
Many of the startling effects of sleight of hand are undoubtedly due in
part to this principle. If two successive actions or sets of
circumstances to which the attention of the spectator is specially
directed follow one another by a very narrow interval of time, they
easily appear continuous, so that there seems absolutely no time for the
introduction of an intermediate step.[24]

There is another limit to sensibility which is in a manner the opposite
to the one just named. It is a law of nervous stimulation that a
continued activity of any structure results in less and less psychic
result, and that when a stimulus is always at work it ceases in time to
have any appreciable effect. The common illustration of this law is
drawn from the region of sound. A constant noise, as of a mill, ceases
to produce any conscious sensation. This fact, it is plain, may easily
become the commencement of an illusion. Not only may we mistake a
measure of noise for perfect silence,[25] we may misconceive the real
nature of external circumstances by overlooking some continuous
impression.

Curious illustrations of this effect are found in optical illusions,
namely, the errors we make respecting the movement of stationary objects
after continued movement of the eyes. When, for example, in a railway
carriage we have for some time been following the (apparent) movement of