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.