Dictionary of Theories, Laws, and Concepts in Psychology

TALBOT–PLATEAU LAW. = Talbot’s law. Named in honor of the English physicist William H. F. Talbot (1800–1877) and the Belgian physicist Joseph A. 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 disc, which is a white disc 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.). When rotated, this disc extinguishes the flicker sensations and shows a uniform gray color. Thus, an intermittent stimulus may be seen as continuous. The effect may also be demonstrated by interrupting a beam of light with a rotating disc (Postman & Egan, 1949). The disc, 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% of the time, and the disc is rotated slowly so the light is interrupted only two or three times a second, the observer sees the light as interrupted (i.e., as alternations of light and dark). However, as the rotation speed of the disc 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 disc. The Talbot–Plateau law was challenged by Fick (1863) and Grunbaum (1898), 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 (see Hyde, 1906; Hecht & Wolf, 1932; Wolf & Zerrahn-Wolf, 1935; cf: Arnold, 1934). The validity of the Talbot–Plateau law

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