_Exceptional External Arrangements._

Passing now to those cases where the exceptional circumstance is
altogether exterior to the organ, we find a familiar example in the
illusions connected with the action of well-known physical forces, as
the refraction of light, and the reflection of light and sound. A stick
half-immersed in water always _looks_ broken, however well we may know
that the appearance is due to the bending of the rays of light.
Similarly, an echo always sounds as though it came from some object in
the direction in which the air-waves finally travel to the ear, though
we are perfectly sure that these undulations have taken a circuitous
course. It is hardly necessary to remind the reader that the deeply
organized tendency to mistake the direction of the visible or audible
object in these cases has from remote ages been made use of as a means
of popular delusion. Thus, we are told by Sir D. Brewster, in his
entertaining _Letters on Natural Magic_ (letter iv.), that the concave
mirror was probably used as the instrument for bringing the gods before
the people. The throwing of the images formed by such mirrors upon smoke
or against fire, so as to make them more distinct, seems to have been a
favourite device in the ancient art of necromancy.

Closely connected with these illusions of direction with respect to
resting objects, are those into which we are apt to fall respecting the
movements of objects. What looks like the movement of something across
the field of vision is made known to us either by the feeling of the
ocular muscles, if the eye follows the object, or through the sequence
of locally distinct retinal impressions, if the eye is stationary. Now,
either of these effects may result, not only from the actual movement of
the object in a particular direction, but from our own movement in an
opposite direction; or, again, from our both moving in the first