SPEECH THEORIES.

TALBOT-PLATEAU LAW. = Talbot's law. Named in honor of the English physicist William Henry Fox Talbot (1800-1877) and the Belgian physicist Joseph Antoine Ferdinand Plateau (1801-1883), this generalized principle states that when a periodic visual stimulus is repeated - at a rate that is adequately high so that to an observer it appears to be fused - it will match in brightness a steady light that has the same time-average luminance. For instance, if the flickering light consists of equally long "on" and "off" periods, the steady state will have one-half the brightness (called Talbot brightness) of the "on" phase. The Talbot-Plateau law is demonstrated by use of the Talbot-Plateau disk that is a white disk with concentric bands, each band showing black and white alternately, but with the same quantity of black and white, divided differently in each band (1/1, 2/2, 4/4, etc.) [cf., Maxwell disk/color-mixing wheel - named after the Scottish physicist James Clerk Maxwell (1831-1879) - is a rotating disk onto which radially slit, overlapping disks of paper/plastic may be mounted to divide the total surface with sections of differing amounts of colors for studying color-mixing effects]. When rotated, the Talbot-Plateau disk extinguishes the flicker sensations and shows a uniform gray color. Thus, an intermittent stimulus may be seen as continuous. The effect may be demonstrated, also, by interrupting a beam of light with a rotating disk. The disk, in this case, has segments cut out of it so that part of the time the light may pass through. Under conditions where the light passes only 50-percent of the time, and the disk is rotated slowly so the light is interrupted only two or three times a second, the observer sees the light as interrupted (i.e., seen as alternations of light and dark). However, as the rotation speed of the disk slowly increases, a point is reached at which the light appears as continuous, and the brilliance of the continuous light will be the same as if the total amount of light had been distributed uniformly over a whole revolution of the disk. The Talbot-Plateau law was challenged by A. Fick and O. Grunbaum, who noted that it may not hold under special circumstances such as high intensity and conditions where fusion of a peripheral field occurs when the eye is fixated. However, the law's validity is now generally accepted under most conditions. The validity of the TalbotPlateau law has an interesting implication for the functional nature of the visual system: it suggests that the response of this system is proportional to stimulus luminance in those regions that precede the location where fusion occurs. See also FERRY-PORTER LAW; VISION/SIGHT, THEORIES OF. REFERENCES

Philosophical Transactions of the Royal Society of London, 3, 298. Plateau, J. A. F. (1835). Sur un principe de photometrie. Bulletins de l'Academie Royale des Sciences de Bruxelles, 2, 52-59. Fick, A. (1863). Uber den zeitlichen verlauf der erregung in der netzhaut. Archiv fur Anatomie und Physiologie, 22, 739-764.

Plateau, J. A. F. (1872). Sur la mesure des sensations physiques, et sur la loi qui lie l'intensite de ces sensations a l'intensite de la cause excitante. Bulletin, Royal Academie/Scien-ces/Lettres/Beaux-Arts/Belgium, 33, 376-388.

Grunbaum, O. (1898). On the intermittent stimulation of the retina. Journal of Physiology, 22, 433-450. Arnold, W. (1934). On the theoretical significance of Talbot's law. Journal of General Physiology, 17, 97-101.

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