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 direction, the object more rapidly than ourselves; or, finally, from our both moving in an opposite direction to this, ourselves more rapidly than the object.

There is thus always a variety of conceivable explanations, and the action of past experience and association shows itself very plainly in the determination of the direction of interpretation.

Thus, it is our instinctive tendency to take apparent movement for real movement, except when the fact of our own movement is clearly present to consciousness, as when we are walking, or when we are sitting behind a horse whose movement we see.