In any consideration of the effect of an increase in the intensity of light on the visibility of objects the following principles should be borne in mind. Roughly speaking, objects are seen because of their size and their difference from the background. With a given physical difference from the background, i. e., a given difference in the coefficient of reflection, the apparent or sensation difference increases rapidly with an increase in the intensity of light. This difference is greater in the case of white on black or light objects on a dark background than for black on white or dark objects on a light background. Whether there is also an effect on the apparent size of the object has not yet been fully determined. It is well known that an increase in the size of the object functions in some unknown relation as an increase of brightness, but the converse