It follows from this fact of the reverberation of the nervous structures 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 objects, as trees, etc., and turn our eyes to an apparently stationary object, as the carpet of the compartment, this seems to move in the contrary direction to that of the trees.  Helmholtz’s explanation of this illusion is that when we suppose that we are fixing our eye on the carpet we are really continuing to move it over the surface by reason of the organic tendency, already spoken of, to go on doing anything that has been done.  But since we are unaware of this prolonged series of ocular movements, the muscular feelings having become faint, we take the impression produced by the sliding of the picture over the retina to be the result of a movement of the object.[26]