There is no general agreement regarding the relative importance of the retinal and the central components in the leveling of sensibility to different intensities of light which is called light and dark adaptation. Although it is probable that in the healthy person light is of more significance than area, the relative importance for acuity of vision of increase in illumination and increase in area merits further study. When vision is impaired as the result of a retinal lesion, it may be that the relative importance of light and of area is altered. When studied by functional tests, the diminution of vision due to papilledema may differ from that due to primary atrophy of the optic nerve, and the relative importance for acuity of vision of light and of area may be altered in cases of lesions in the visual pathways or in the occipital lobes of the brain. Furthermore, in