Action of a Light-Source in the Field of View in Lowering Visibility

In the first part of this article, results of an experimental investigation of the action of a light-source in the field of view in changing the adaptation of the fovea of the eye, and in increasing the minimum perceptible brightness difference are presented. In the second part are presented results of a study to determine whether a given light-source in the field of view is an advantage or disadvantage; and also a method is presented for expressing visibility by an exact ratio.