The course of dark adaptation following pre-exposures of all combinations of 4 intensities and 4 durations was measured on two human subjects. Initial threshold rises as a function of increased pre-exposure intensity or duration. "When the pre-exposure intensity is increased by a given factor, the initial threshold value is raised more than if the duration is increased by the same factor." The test stimulus was violet, and the last report of color was at lower intensity values when the pre-exposure was of lower intensity. (PsycINFO Database Record (c) 2006 APA, all rights reserved)