Abstract

Dark adaptation functions were determined by presenting a 2° square test field in the temporal parafovea, or a 1.3° field in the fovea, against backgrounds or surround luminances varying in brightness from complete darkness to 40 millilamberts. The test field was composed of parallel, equally spaced dark and light vertical bars, and presented in the form of flashes of 0.04 sec. As surround luminance is systematically increased the total extent of adaptation, the relative participation of rods and cones, and the transition from cone to rod function vary as a function of background luminance. Pre-exposure to a light source emitting radiation from 2850 to 4100 angstroms in addition to white light has an inhibitory effect on the sensitivity of the retinal receptors which is evident in higher excitation thresholds as compared with those obtained when the ultraviolet is screened from the pre-exposure source. The ultraviolet action which previously was only shown for the rods is also found in foveal and parafoveal cones, when, owing to the presence of a surround luminance, the rods are prevented from entering into action, and the range of adaptation is limited to levels above threshold for cone excitation.

© 1951 Optical Society of America

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