Recent studies in sensory physiology have provided a new approach to the problem of the mechanism of sense organs. The discharge of nerve impulses in the afferent fibers from various receptors has been studied in preparations in which the activity can be limited to a single end organ and its attached nerve fiber. The more complete analysis characteristic of this approach is best exemplified in the work done on tension, touch, and pressure receptors (Adrian, '26; Adrian and Zotterman, '26 ; Bronk, '29 ; Matthews, '31 ; Adrian, Cattell, and Hoagland, '31; Adrian and Umrath, '29; Bronk and Stella, '32). In the case of these relatively simple end organs

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Recent discussions of intensity discrimination in vision have focussed attention on initial events in the process. Hecht (1935), in particular, has proposed a theory which states that brightness discrimination is due to the photochemical processes which take place at the initial moment when the eye, already adapted to a given intensity, is exposed to a just discriminahly higher intensity. Results of recent observations by Smith (1936) and Steinhardt (1936) lend support to the hypothesis.An important question which arises when we consider a theory in terms of initial events is the problem of how such a formulation may be related to the Bunsen-Roscoe law. This law states that, for brief flashes of light, the product of intensity and duration is constant for the production of a constant photochemical effect. It has been found to apply within well marked limits of exposure to both the fovea and periphery of the human eye, for threshold (Graham and Margaria, 1935;Karn, 1936) and supraliminal excitation (McDougall, 1904;Blondel and Rey, 1911;Graham and Cook, 1937). Adrian and Matthews (1927) and Hartline (1928) have demonstrated the law for the eyes of lower organisms, and Hartline's findings for the single fiber of Limulus (1934) give adequate evidence on the nature and limitations of its application. The strict reciprocity relation fails for exposures longer than a "critical duration" beyond which, in the Limulus eye and probably in the human eye (Karn, 1936), the relation I. t = Constant is superseded by the relation I = Constant.These considerations have led us to perform the experiments reported here. It has seemed important to us, because of the emphasis on initial events in brightness discrimination, to determine the effect 635

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