'Swindle's Ghost' is a term for an optical illusion that some psychologists have offered as a possible scientific explanation for ghost sightings. Actually, I doubt that many sightings are a result of this phenomenon, but I like the name.
Special Correspondent Paul Brock (May 15, 1967) offered this explanation:
One after-image, which psychologists believe has given rise to many reports of nocturnal apparitions, is called "Swindle's Ghost." It was first described by the American psychologist P.F. Swindle, about 45 years ago.
It can be summoned up by anybody. Using no more ectoplasm than a table lamp, friends can join you in this weird experiment, right in your own living room. Choose a dark moonless night and draw the drapes securely so that no stray light from street lamps or passing cars enters the room. Group the chairs near a table or floor lamp with one person directly alongside it to switch it on and off.
First, everyone must remain in the darkened room for at least 10 minutes before the experiment begins, so that the eyes can adjust completely to the darkness. Then, each ghosthunter must look steadily toward the lamp but not directly at it. They must keep perfectly still and keep the eyes from moving during the time the room will be illuminated and immediately afterward.
Now turn on the lamp for a full second. Turn it off. Shortly after you will see the whole scene loom up in the darkness with startling clarity, and the ghost impression will last for some time. Not only will everything appear exactly as it was when the light was on, but many precise details will be evident which could not possibly have been noted during the brief illumination...
The same optical illusion occurs when someone reports that he has seen a ghost in a graveyard at night. If a man is passing a graveyard at night and the moon breaks through the clouds just as he is opposite a white tombstone, in a few minutes he might see a vague white form loom up before him. The moon's illumination has created the 'ghost' which the man actually does see, but which is only an after-image— in the image of "Swindle's Ghost."
Some more info in the book Systems Theories and A Priori Aspects of Perception:
Swindle observed that if one experiences a very bright flash, one achieves a very powerful positive afterimage that may last over a period of hours. "Swindle's Ghost," as it is referred to occasionally, is a conscious image sustained purely by cortical activity; it is an image created by a stimulus that is not present during later observation. In spite of the absence of a distal stimulus, the image is very real. Observations by Gregory, Wallace, and Campbell (1959) and Davies (1973) attests to just how real it is; if a Swindle's Ghost image is a corridor, and one walks down it in total darkness, one seems to be walking, briefly, through his or her own afterimage.
You can find Swindle's original article here:
Swindle, P.F. (1916). Positive Afterimages of long duration. American Journal of Psychology, 27, 325-334.