Persistence of vision refers to the optical illusion that occurs when visual perception of an object does not cease for some time after the rays of light proceeding from it have ceased to enter the eye. The illusion has also been described as "retinal persistence", "persistence of impressions", simply "persistence" and other variations.
This has been believed to be the explanation for motion perception in cinema and animated films, but this theory has long been discarded by scientists.
It is assumed that the illusion that film historians often refer to as "persistence of vision" is the same as what is known as positive afterimages by psychologists. The cause of positive afterimages is not well known, but possibly reflects persisting activity in the brain when the retinal photoreceptor cells continue to send neural impulses to the occipital lobe.
Early descriptions of the illusion often attributed the effect purely to imperfections of the eye, particularly of the retina. Nerves and parts of the brain later became part of explanations.
Some natural phenomena and the principles of some optical toys have been attributed to the persistence of vision effect.
The fact that a glowing coal appears as a line of light when it is moved around quickly has been used to illustrate the idea of persistence of vision. It is known as the "sparkler's trail effect", named after the trail that appears when a sparkler is moved around quickly.
The effect has been applied in the arts by writing or drawing with a light source recorded by a camera with a long exposure time.
Colors on spinning tops or rotating wheels mix together if the motion is too fast to register the details. A colored dot then appears as a circle and one line can make the whole surface appear in one uniform hue.
The Newton disc optically mixes wedges of Isaac Newton's primary colors into one (off-)white surface when it spins fast.
In April 1825 the first Thaumatrope was published by W. Phillips (in anonymous association with John Ayrton Paris). The fact that the image of one side of the disc seems to blend with the image of the other side when it is looked at while it is twirled very fast, is often used as an illustration of persistence of vision.
In April 1858 John Gorham patented his Kaleidoscopic colour-top. This is a top on which two small discs are placed, usually one with colors and a black one with cut-out patterns. When the discs spin and the top disc is retarded into regular jerky motions the toy exhibits "beautiful forms which are similar to those of the kaleidoscope" with multiplied colours. Gorham described how the colours appear mixed on the spinning top "from the duration of successive impressions on the retina". Gorham founded the principle on "the well-known experiment of whirling a stick, ignited at one end" (a.k.a. the sparkler's trail effect).
A pencil or another rigid straight line can appear as bending and becoming rubbery when it is wiggled fast enough between fingers, or otherwise undergoing rigid motion.
Persistence of vision has been discarded as sole cause of the illusion. It is thought that the eye movements of the observer fail to track the motions of features of the object.
This effect is known as an entertaining "magic" trick for children.
The term "persistence of vision display" or "POV display" has been used for LED display devices that compose images by displaying one spatial portion at a time in rapid succession (for example, one column of pixels every few milliseconds). A two-dimensional POV display is often accomplished by means of rapidly moving a single row of LEDs along a linear or circular path. The effect is that the image is perceived as a whole by the viewer as long as the entire path is completed during the visual persistence time of the human eye. A further effect is often to give the illusion of the image floating in mid-air. A three-dimensional POV display is often constructed using a 2D grid of LEDs which is swept or rotated through a volume. POV display devices can be used in combination with long camera exposures to produce light writing.
Although the theory of persistence of vision as the (main) reason we see film as motion has been disproved since 1912, film historians have persisted in citing the theory as well as historical references to afterimages and similar illusions. The following developments are relevant to that story.
Aristotle (384-322 BCE) noted that the image of the sun remained in his vision after he stopped looking at it.
The discovery of persistence of vision is sometimes attributed to the Roman poet Lucretius (c. 15 October 99 BCE - c. 55 BCE), although he only mentions it in connection with images seen in a dream.
Around 165 CE Ptolemy described in his book Optics a rotating potter's wheel with different colors on it. He noted how the different colors of sectors mixed together into one color and how dots appeared as circles when the wheel was spinning very fast. When lines are drawn across the axis of the disc they make the whole surface appear to be of a uniform color. "The visual impression that is created in the first revolution is invariably followed by repeated instances that subsequently produce an identical impression. This also happens in the case of shooting stars, whose light seems distended on account of their speed of motion, all according to the amount of perceptible distance it passes along with the sensible impression that arises in the visual faculty."
Porphyry (circa 243-305) in his commentary on Ptolemy's Harmonics describes how the senses are not stable but confused and inaccurate. Certain intervals between repeated impressions are not detected. A white or black spot on a spinning cone (or top) appears as a circle of that color and a line on the top makes the whole surface appear in that color. "Because of the swiftness of the movement we receive the impression of the line on every part of the cone as the line moves."
In the 11th century Ibn al-Haytam, who was familiar with Ptolemy's writings, described how colored lines on a spinning top could not be discerned as different colors but appeared as one new color composed of all of the colors of the lines. He deducted that sight needs some time to discern a color. al-Haytam also noted that the top appeared motionless when spun extremely quick "for none of its points remains fixed in the same spot for any perceptible time".
Leonardo da Vinci was also curious about the phenomenon.
Isaac Newton (1642-1726/27) purportedly demonstrated how white light is a combination of different colors with a rotating disc with color segments. When spinning fast the colors seem to blend and appear as white (or rather an off-white light hue). For an explanation he referred to the sparkler's trail effect: a gyrating burning coal could appear as a circle of fire because "the sensation of the coal in the several places of that circle remains impress'd on the sensorium, until the coal return again to the same place."
In 1821 the Quarterly Journal of Science, Literature, and The Arts published a "letter to the editor" with the title Account of an Optical Deception. It was dated Dec. 1, 1820 and attributed to "J.M.", possibly publisher/editor John Murray himself. The author noted that the spokes of a rotating wheel seen through fence slats appeared with peculiar curvatures (see picture). The letter concluded: "The general principles on which this deception is based will, immediately occur to your mathematical readers, but a perfect demonstration will probably prove less easy than it appears on first sight". Four years later Peter Mark Roget offered an explanation when reading at the Royal Society on December 9, 1824. He added: "It is also to be noticed that, however rapidly the wheel revolves, each individual spoke, during the moment it is viewed, appears to be at rest." Roget claimed that the illusion is due to the fact "that an impression made by a pencil of rays on the retina, if sufficiently vivid, will remain for a certain time after the cause has ceased." He also provided mathematical details about the appearing curvatures.
As a university student Joseph Plateau noticed in some of his early experiments that when looking from a small distance at two concentric cogwheels which turned fast in opposite directions, it produced the optical illusion of a motionless wheel. He later read Peter Mark Roget's 1824 article and decided to investigate the phenomenon further. He published his findings in Correspondance Mathématique et Physique in 1828 and 1830. In 1829 Plateau presented his then unnamed anorthoscope in his doctoral thesis Sur quelques propriétés des impressions produites par la lumière sur l'organe de la vue. The anorthoscope was a disc with an anamorphic picture that could be viewed as a clear immobile image when the disc was rotated and seen through the four radial slits of a counter-rotating disc. The discs could also be translucent and lit from behind through the slits of the counter-rotating disc.
On December 10, 1830 scientist Michael Faraday presented a paper at the Royal Institution of Great Britain, titled On a Peculiar Class of Optical Deceptions, about the optical illusions that could be found in rotating wheels. He referred to Roget's paper and described his relating new findings. Much was similar to what Plateau had already published and Faraday later acknowledged this. However, a unique part of Faraday's experiments concerned turning wheels in front of a mirror and this inspired Plateau with the idea for new illusions. In July 1832 Plateau sent a letter to Faraday and added an experimental circle that produced a "completely immobile image of a little, perfectly regular horse" when rotated in front of a mirror. After several attempts and many difficulties Plateau managed to animate the figures between the slits in a disc when he constructed the first effective model of the phénakisticope in November or December 1832 . Plateau published his then unnamed invention in a January 20, 1833 letter to Correspondance Mathématique et Physique.
Simon Stampfer independently and almost simultaneously invented his very similar Stroboscopischen Scheiben oder optischen Zauberscheiben (stroboscopic discs or optical magic discs) soon after he read about Faraday's findings in December 1832.
Stampfer also mentioned several possible variations of his stroboscopic invention, including a cylinder (similar to the later zoetrope) as well as a long, looped strip of paper or canvas stretched around two parallel rollers (somewhat similar to film) and a theater-like frame (much like the later praxinoscope). In January 1834, William George Horner also suggested a cylindrical variation of Plateau's phénakisticope, but he did not manage to publish a working version. William Ensign Lincoln invented the definitive zoetrope with exchangeable animation strips in 1865 and had it published by Milton Bradley and Co. in December 1866.
The idea that the motion effects in so-called "optical toys", like the phénakisticope and the zoetrope, is caused by images lingering on the retina was questioned in an 1868 article by William Benjamin Carpenter. He suggested that the illusion was "rather a mental than a retinal phenomenon".
Narrowly defined, the theory of persistence of vision is the belief that human perception of motion (brain centered) is the result of persistence of vision (eye centered). That version of the theory was discarded well before the invention of film and also disproved in the context of film in 1912 by Wertheimer but persists in citations in many classic and modern film-theory texts. A more plausible theory to explain motion perception (at least on a descriptive level) are two distinct perceptual illusions: phi phenomenon and beta movement.
A visual form of memory known as iconic memory has been described as the cause of this phenomenon. Although psychologists and physiologists have rejected the relevance of this theory to film viewership, film academics and theorists generally have not. Some scientists nowadays consider the entire theory of iconic memory a myth.
When contrasting the theory of persistence of vision with that of phi phenomena, an understanding emerges that the eye is not a camera and does not see in frames per second. In other words, vision is not as simple as light registering on a medium, since the brain has to make sense of the visual data the eye provides and construct a coherent picture of reality. Joseph Anderson and Barbara Fisher argue that the phi phenomena privileges a more constructionist approach to the cinema (David Bordwell, Noël Carroll, Kirstin Thompson) whereas the persistence of vision privileges a realist approach (André Bazin, Christian Metz, Jean-Louis Baudry).
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