The following account may be taken as typical of the true illusion:[1]
"One evening, at dusk, I went into my bedroom to fetch something I wanted off the mantelpiece. A street lamp threw a slanting ray of light in at the window, just sufficient to enable me to discern the dim outline of the chief articles of furniture in the room. I was cautiously feeling for what I wanted when, partially turning round, I perceived at a short distance behind me the figure of a little old lady, sitting very sedately with her hands folded in her lap, holding a white pocket handkerchief. I was much startled, for I had not before perceived any one in the room, and called out 'Who's that?' but received no answer, and, turning quite round to face my visitor, she immediately vanished from sight. 'Well' I thought, 'this is strange!' I had left all the rest of the household downstairs; it was hardly possible that any one could have followed me into the room without my being aware of it, and besides, the old lady was different from any one I had ever seen. Being very near-sighted, I began to think my eyes had played me a trick; so I resumed my search in as nearly as possible the same position as before, and having succeeded, was turning to come away, when lo! and behold! there sat the little old lady as distinct as ever, with her funny little cap, dark dress, and hands folded demurely over her white handkerchief. This time I turned round quickly and marched up to the apparition, which vanished as suddenly as before. And now being convinced that no one was playing me any trick, I determined to find out, if possible, the why and because of the mystery. Slowly resuming my former position by the fireplace, and again perceiving the figure, I moved my head slightly from side to side, and found that it did the same. I then went slowly backward, keeping my head still until I reached the place, when, deliberately turning round, the mystery was solved. A small polished mahogany stand near the window, which I used as a cupboard for various trifles, made the body of the figure, a piece of paper hanging from the partly opened door serving as the handkerchief; a vase on the top made the head and headdress, and the slanting light falling upon it and the white curtain of the window completed the illusion. I destroyed and remade the figure several times and was surprised to find how distinct it appeared when the exact relative positions were maintained."
In this case the form of the illusion seems to have been almost entirely determined by sensory stimuli, but in many cases the operation of the central factor can still be traced. For example, Parish quotes[2] from Prof. Lazarus an experience of his own.
