Were the eye in the portrait a real eye, a side movement of the spectator would, it is evident, cause him to see less of the pupil and more of the side of the eyeball, and he would only continue to see the full pupil when the eye followed him.

We regard the eye in the picture as a real eye having relief, and judge accordingly.
We may fall into similar illusions respecting distance in auditory perception.

A change of wind, an unusual stillness in the air, is quite sufficient to produce the sense that sounding objects are nearer than they actually are.

The art of the ventriloquist manifestly aims at producing this kind of illusion.

By imitating the dull effect of a distant voice, he is able to excite in the minds of his audience a powerful conviction that the sounds proceed from a distant point.