nervous elements further to the right. And since, in this instance, the
displacement is not produced in the ordinary way by the activity of the
ocular muscle making itself known by certain feelings of movement, it is
disregarded altogether, and the direction of the objects is judged as
though the eye were stationary.

A somewhat similar illusion as to direction occurs in auditory
perception. The sense of direction by the ear is known to be due in part
to the action of the auricle, or projecting part of the ear. This
collects the air-waves, and so adds to the intensity of the sounds,
especially those coming from in front, and thus assists in the
estimation of direction. This being so, if an artificial auricle is
placed in front of the ears; if, for example, the two hands are each
bent into a sort of auricle, and placed in front of the ears, the back
of the hand being in front, the sense of direction (as well as of
distance) is confused. Thus, sounds really travelling from a point in
front of the head will appear to come from behind it.

Again, the perception of the unity of an object is liable to be
falsified by the introduction of exceptional circumstances into the
sense-organ. This is illustrated in the well-known experiment of
crossing two fingers, say the third and fourth, and placing a marble or
other small round object between them. Under ordinary circumstances, the
two lateral surfaces (that is, the outer surfaces of the two fingers)
now pressed by the marble, can only be acted on simultaneously by _two_
objects having convex surfaces. Consequently, we cannot help feeling the
presence of two objects in this exceptional instance. The illusion is
analogous to that of the stereoscope, to be spoken of presently.