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.