Using thoughts to cause photographic images to appear on film.
This goofy idea was made popular by a psychiatrist, Dr. Jule Eisenbud, who wrote a book about Ted Serios, who claimed he could make images appear on Polaroid film just by thinking of an image. How he accomplished this `magic' is described by James Randi.
further reading
Randi, James. Flim-Flam! (Buffalo, New York: Prometheus Books,1982), pp. 222ff.