Abstract
It has been long known by lighting engineers that a shift in eye movement from a dark to light surface and back again has a strong (and deleterious) effect upon visual acuity. Engineers faced with the task of designing efficient offices, adequate school buildings, and safe and productive industrial plants have traditionally paid a good deal of attention to brightness ratios between task and surrounding environment. The study described here is a step toward the more exact determination of what these ratios should be. The work to be described in this report concerns the visibility changes produced by sudden increases or decreases in a prevailing luminance level.