FOVEAL DISCRIMINATION MEASURED WITH TWO SUCCESSIVE LIGHT FLASHES

The flicker-fusion frequency is defined as the number of successive light flashes per second at which the impression of flicker ceases and is replaced by an impression of steady light. This frequency then indicates the briefest dark interval discerned under the given conditions. The flicker-fusion frequency represents a complex function and depends on a number of factors, int. al. luminance, area, and exposure time (Simonson & Brozek, 1952; Berger & Mahneke, 1952; Landis, 1954). Of these factors the relation to the luminance of the test patch has been most thoroughly investigated. The first systematic experiments by Ferry (1892) and Porter (1898) revealed that there is a linear relationship between the flickerfusion frequency and the logarithm of the luminance (Ferry-Porter's law). These investigators, thus, determined the relation between the quantity of light and the briefest dark interval between a large number of successive flashes that the human visual organ could discriminate or fuse. Subsequent studies (Hecht & Verrijp, 1934; Crozier 8c Wolff, 1941; Kugelmass & Landis, 1954) have shown that this law holds only for medium luminances and areas of from approximately 1-14'. Gildemeister (1914) and Dunlap (1915) studied a similar relationship, viz. that of the quantity of light and the dark interval between only two successive flashes. Gildemeister varied the luminance of the flashes. Dunlap altered their duration, but doubted whether his experiments could be reproduced for technical reasons. It, therefore, seemed worthwhile to repeat Dunlap's experiments, using an improved technique. Since an observer will experience the same phases during

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