'Two main theories have been suggested to explain the fact that intermittent retinal stimuli repeated above a certain frequency give rise to a steady sensation.

The older, which I shall in future style the "persistence theory," seems to have been suggested by d'Arcy [note: 'Mem. Akd. Sci. p. 439. 1765.'] in 1765 and maintains that a steady sensation results from intermittent stimuli, when the intervening periods do not exceed the time of duration of the positive after-image of undiminished brightness: in fact, attempts were made by this observer to determine this quantity by noting the minimum freouency with which a burning coal must be rotated in order to produce a sensation of a ring of fire: 0.133 secs. was his estimation.

All subsequent writers, Plateau, Talbot, Helmholtz, etc. adopt d'Arcy's view, Fick [note: Hernann's Handbuch, III. p. 215. 1879.'] being the first to suggest an alternative.' (443-444)