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.
_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.