1. _Affections of the organs of special sense._--Prominent among these is the effect of light upon the eye. The darkness of night favors sleep; the presence of light hinders its incidence and renders it less profound. During the gloom of a total eclipse animals seek their shelter; birds hide themselves in their nests; domestic fowls arrange themselves upon the roost, and seem quite disconcerted by the speedy return of sunlight. Children often find it difficult to sleep in an illuminated room. I have known nurses who would sit with a wakeful infant under a powerful gaslight till after midnight, and then would express their surprise that the baby persisted in gazing at the flame instead of going quietly to sleep. The inhabitants of Northern Europe find it necessary to darken their sleeping rooms during the long polar day; and travelers in such regions often suffer for want of the natural sleep which only darkness affords. Judge Caton, writing of his travels in Norway[28] says: “We longed for darkness and for night. Do what we could to darken the windows to keep out the light, still it was not night as nature makes it, and which the habit of a lifetime had rendered necessary to sound repose. Artificial darkness, especially when incomplete, is as far from night as artificial light is from day.... These sunny nights can hardly conduce to health, they steal away so much of sleep. One does not readily get sleepy in the sunshine, and then we are so apt to forget to look at the watch to see if it is time to retire.” In the tropical regions of the world it is usual for the inhabitants to sleep during the middle of the day; but they take great pains to exclude the light from their houses during the hours of sleep. The Pacific Islanders cover their faces with the bed clothes for the purpose of excluding the light while attempting to sleep. Repose thus obtained in the daytime often serves to convert the night into a season of wakefulness. The Africans sleep and dream away the heated hours of the day, and give up considerable portions of the night to festivity in the open air--a practice which undoubtedly contributes to the permanence of an inferior grade of social life. Sudden illumination of the sleeping room will frequently awaken the sleeper. During the great fire in Chicago, A. D. 1871, many persons were thus aroused from their slumbers as the flames lighted up the streets adjoining their houses. One of my acquaintances was awakened one night by a flash of light from the lantern of a burglar who was moving noiselessly about her chamber. The experience of almost every one will testify to the effects of sheet-lightning silently illuminating the sky by night. Dreams, also, are not unfrequently excited by the incidence of light upon the closed eyelids.[29]