BLINDNESS, PERIODICAL, is that which comes and goes by turns, according to the season of the moon, the time of day, and the like. Diurnal blindness is called hemeralopia. Nocturnal blindness, called also nyctalopia, is that which ensues on the setting of the sun in persons who see perfectly in the day, but become quite blind at night approaches. (See Phil. Trans. No. 159, p. 560; and a singular case related by Dr Samuel Pye in the Medical Obsrv. and Inquir. vol. i. p. 111.)